31    fill 


Iron  Tannage 


DISSERTATION 

SUBMITTED  IN  PARTIAL  FULFILMENT  OF  THE  REQUIREMENT  FOR 

THE  DEGREE  OF  DOCTOR  OF  PHILOSOPHY  IN  THE 

FACULTY  OF  PURE  SCIENCE,  COLUMBIA 

UNIVERSITY  IN  THE  CITY  OF 

NEW  YORK 


BY 


Te-Pang  Hou,  S.  B.,  M.  A. 


NEW  YORK,  U.  S.  A. 
1921 


-00 


EXCHANGE 


To  AI.LKN  ROGERS  to  whom  the  author  owes  his  first  interest 
in  tanning  this  work  is  dedicated. 


H 


^52335 


SPECIAL  ACKNOWLEDGMENTS. 

The  author  desires  to  express  his  indebtedness  to  Professor 
Daniel  Dana  Jackson,  head  of  the  Chemical  Engineering  Depart- 
ment, Columbia  University,  without  whose  guidance  and  gen- 
erous help  this  work  could  not  have  been  successfully  carried 
out. 

To  Dr.  Allen  Rogers,  Pratt  Institute,  Brooklyn,  N.  Y.,  he 
wishes  to  express  his  thanks  for  the  helpful  suggestions  and  for 
the  use  of  the  tannery  under  his  charge  for  fully  one-half  of  a 
year. 


CONTENTS 


PAGE 

I 


SECTION  I.  General  Discussion 

SECTION  II.          History  of  Iron  Tannage  Including  Recent  Work        5 

SECTION  III.  Investigation  of  Different  Methods  of  Oxidation 
and  Some  Methods  of  Preparation  of  Iron 
Liquor  from  Copperas  14 

SECTION  IV.         Hydrolysis    and    Decomposition    of    Ferric    and 

Chromic  Salts  Compared  28 

SECTION  V.          On  the  Relation  of  Basicity  to  Stability  in  Iron 

Liquor  35 

SECTION  VI.         Behavior  of  the   Pelt  towards   Iron  Tan  Liquor      43 

SECTION  VII.       Experiments  on  Tanning  with  Ferric  Hydroxide 

Hydrosol  51 

SECTION  VIII.  General   Experimental  Work  on  Iron  Tanning  54 

SECTION  IX.  Chrome  Iron  Joint  Tannage  59 

SECTION  X.  Pure  Iron  Tannage  64 

SECTION  XI.  Iron  Phosphate  Tannage  69 

SECTION  XII.  Conclusions  72 

BIBLIOGRAPHY  The  More  Important  Works  in  Leather  Industry  74 

APPENDIX  A  Tentative  Procedure  for  the  Ordinary  Chemical 

Analysis  of  Iron-Tanned  Leather  77 


[REPRINTED  FROM  JOURNAL  OF  AMERICAN  LEATHER' CHEMISTS 
ASSOCIATION,  FEBRUARY,  MARCH,  APRIL.  MAY,  1921.] 


IRON  TANNAGE. 

SECTION  I.    GENERAL  DISCUSSION.' 

As  early  as  the  latter  half  of  the  eighteenth  'century  iron  salts 
as  tanning  agents  were  proposed  and  experimented  upon.  From 
that  time  on  attempt  after  attempt  was  made  to  obtain  a  satis- 
factory tannage  with  iron,  but  without  material  results.  Differ- 
ent experimenters  took  up  the  matter  with  renewed  enthusiasm, 
each  cherishing  a  hope  of  thereby  revolutionizing  the  tanning  in- 
dustry, but  plants  running  on  such  processes  were  unsuccessful. 
When  it  is  considered  that  next  to  alum,  iron  compounds  were 
among  the  first  inorganic  salts  introduced  for  mineral  tannage,  it 
may  be  realized  how  far  iron  tannage  has  fallen  behind  that  of 
chrome  and  even  of  alum  tannage  at  the  present  time.  Diversified 
reasons  have  been  given  by  different  investigators  to  account  for 
the  unsatisfactory  products  obtained.  No  doubt,  while  some  of 
these  represent  true  causes  responsible  for  the  failure,  much  is 
due  to  a  lack  of  understanding  of  the  peculiar  properties  of  the 
iron  salts  rather  than  to  the  intrinsic  character  of  iron.  It  has 
been  our  object  to  make  a  study  of  iron  tannage  and  to  determine 
wherein  the  difficulties  lie  and  how  they  may  be  met. 

True  to  the  general  trend  of  the  Periodic  Table,  aluminum  with 
the  lowest  atomic  weight  is  the  most  acidic,  while  iron  having  the 
highest  atomic  weight  is  the  most  basic,  with  chromium  lying  in 
between.1  This  accounts  for  some  of  the  differences  in  the  be- 
havior of  the  three  elements  as  tanning  agents.  But  there  seem 
to  be,  as  far  as  the  tanning  properties  are  concerned,  more  funda- 
mental differences,  not  in  degree  but  in  kind,  which  should  be 
ascribed  to  their  distinct  properties  as  individual  elements.  For 
instance,  both  iron  and  alum  tanned  leathers  do  not  resist  boiling 
temperature,  whereas  chrome  tanned  leather  is  not  affected  unless 
it  is  subjected  to  boiling  continuously  for  a  considerable  length  of 
time. 

Some  investigators  seem  to  have  worked  along  the  idea  that 

the  basic  ferric  sulphate  corresponding  to  the  formula  Fe(OH)- 

SO4  is  the  compound  in  the  liquor  that  brings  about  tanning,  and 

^ee  Stieglitz,  "Quantitative  Chemical  Analysis,"  Vol.  I,  p.  195  (1919). 


2  LEATHER   CHEMISTS   ASSOCIATION 

have  striven  to  produce  such  a  basic  ferric  salt  liquor  for  this 
purpose.  This  is  too  basic  for  a  sulphate  liquor,  although  ferric 
chloride  liquor  can  stand  a  higher  basicity  than  this.  Ferric  sul- 
phate in  solution  corresponding  to  this  degree  of  basicity  is  not 
stable  and  the  hydrated  -  ferric  oxide,  Fe2O3.xH2O,  a  fine  yellow 
crystalline  precipitate,  will  invariably  separate  out  within  a  short 
time,  even  when  the  liquor  is  not  diluted.  If  the  liquor  is  diluted, 
turbidity  is  almost  instantaneous  with  a  rapid  separation  of  the 
light  yellow  precipitate.  To  produce  a  good  tannage  with  iron, 
the  basicity  of  the  liquor  employed  is  found  to  be  considerably 
less  than  this  in  the  case  of  the  sulphate  and  the  range  between 
which  the  basicity  may  vary  is  rather  narrow.  Symbolically  the 
salt  in  the  ferric  sulphate  tan  liquor  may  be  represented  by  Fe2- 
(OH)a..(SO4)>,  where  x  has  a  value  between  i  and  \]/2  and  3; 
between  2^  and  2^/4,  but  there  is  no  definite  formula  assignable, 
as  there  is  no  sharply  defined  chemical  compound  and,  besides, 
when  it  comes  to  neutralization,  the  iron  that  is  fixed  in  the  pelt 
is  of  a  highly  basic  character.  But  any  attempt  to  bring  about 
the  formation  of  such  a  basic  salt  as  is  represented  by  Fe(OH)- 
(SO4)  invariably  causes  a  precipitate  of  Fe2O3.xH2O.  This 
Fe2O3.xH2O  is  entirely  inert  and  we  can  not  expect  to  obtain  a 
tannage  from  it  any  more  than  from  a  precipitated  Cr2O3.xH2O  if 
such  is  produced  in  the  chrome  liquor  from  the  use  of  too  much 
alkali.  The  oft  reported  "hard  and  brittle  leather"  may  be  simply 
due  to  improper  tannage  from  the  presence  of  much  precipitated 
Fe2O3.xH2O.  The  difficulty  with  the  iron  liquor,  then,  is  that  the 
ferric  salt,  particularly  the  sulphate,  is  very  readily  hydrolyzed 
and  that,  what  is  still  worse,  upon  hydrolysis  the  ferric  hydroxide 
seems  to  pass  through  the  colloidal  range  so  rapidly  that  it  is 
soon  flocculated  or  coagulated  as  a  light  yellow  crystalline  sub- 
stance Fe2O3.xH2O.  This  will  be  appreciated  when  it  is  seen  that 
A1(OH)3  upon  hydrolysis  remains  in  the  colloidal  realm  for  a 
considerable  time,  and  when  finally  separated  out  from  the  solu- 
tion it  does  not  readily  become  dehydrated  as  Al2O3.xH2O.  The 
same  is  true  of  Cr(OH)3  from  a  chromic  salt  solution,  and  even 
more  pronounced. 

Another  difficulty  is  that  ferrous  iron  in  solution,  though  easily 
partially  oxidized  is  rather  difficult  to  be  completely  oxidized.  To 
effect  a  partial  oxidation  is  one  thing,  but  to  oxidize  completely  all 


IRON    TANNAGE  3 

the  ferrous  iron  in  solution  to  the  ferric  state  seems  quite  another. 
Oxygen  from  the  air  will  partially  oxidize  some  ferrous  iron  in 
solution  to  the  ferric  state,  but  it  never  is  able  to  oxidize  it  com- 
pletely, except  in  alkaline  solution.  Perchloric  acid,  another  ox-1 
idizing  agent,  oxidizes  iron  in  solution  partially  but  complete  oxi- 
dation is  quite  difficult.  With  I2,  complete  oxidation  is  impossi- 
ble, although  a  small  amount  of  FeI3  is  obtained.  There  is  nor- 
mally always  an  equilibrium  between  the  ferrous  iron  and  the 
ferric  iron  in  solution.  To  shift  the  equilibrium  to  completion 
requires  a  much  higher  potential  which  is  only  secured  by  using 
a  powerful  oxidizing  agent,  or  for  the  same  oxidizing  agent  a 
higher  concentration  of  this.  The  equilibrium  ratio  is 


(Fe"1) 

Thus,  a  small  concentration  of  ferric  iron  present  in  solution  is 
normally  sufficient  to  set  up  an  equilibrium  and  it  takes  a  con- 
siderable potential  to  shift  the  equilibrium  to  the  ferric  end. 
This  consequently  calls  for  the  presence  of  some  excess  of  a 
powerful  oxidizing  agent  to  prevent  any  of  the  ferric  iron  from 
being  again  reduced.  Complexities  result  from  the  fact  that  iron 
is  capable  of  existing  in  two  different  states,  of  which  the  ferric 
state  is  the  one  that  possesses  the  tanning  property  and  this  gives 
rise  to  one  of  the  great  difficulties  in  iron  tannage. 

In  the  case  of  aluminum  the  problem  is  simpler,  as  aluminum 
does  not  exist  in  a  state  other  than  the  trivalent,  and  it  has  little 
or  no  tendency  to  be  flocculated  into  a  similarly  dehydrated 
Al2O3.xH2O  compound  in  the  solution. 

As  to  chromium  the  condition  is,  on  the  other  hand,  decidedly 
favorable.  For,  while  chromium  does  exist  in  other  states,  notably 
as  CrO4~,  or  Cr2O7=,  it  would  take  an  excessively  high  oxidation 
potential  to  cause  it  to  go  to  the  chromate  state,  except  in  an 
alkaline  solution,  which  is  not  the  case  with  the  chrome  bath. 
Hence  to  all  intents  and  purposes  chromium  under  such  condi- 
tions can  be  said  to  possess  only  one  state  of  oxidation,  i.  e.,  Crm, 
as  is  also  the  case  with  Al.  Further,  tanning  conditions  are  far 
more  favorable  to  reduction  than  to  oxidation.  The  protein 
a  See  Stieglitz,  .  "Quantitative  Chemical  Analysis,"  Vol.  I,  pp.  270-275 
(1919). 


4  LEATHER   CHEMISTS   ASSOCIATION 

bodies  in  the  hides  and  skins,  the  woody  material  of  the  drum  or 
paddle,  and  the  metallic  joints  of  the  apparatus'  all  possess  dis- 
tinct reducing  properties.  Ferric  salt  in  the  liquor  which  is  so 
susceptible  of  reduction  is  always  reduced  to  a  greater  or  less  de- 
gree, as  can  be  easily  proved  by  testing  the  spent  liquor  with  potas- 
sium ferricyanide  solution  after  the  tanning  operation.  In  this 
connection  it  can  not  be  too  strongly  emphasized  that  the  tan  liquor 
should  not  be  left  in  contact  with  a  wooden  or  iron  container  un- 
necessarily, and  should  not  be  introduced  into  the  drum  until  the 
tanning  operation  is  ready  to  begin.  Much  trouble  has  been  traced 
to  the  iron  liquor  being  materially  reduced  and  converted  to  the 
ferrous  state  during  the  progress  of  tanning.  For  chrome  tan- 
nage, this  condition  is  exactly  what  we  desire  as  chromium  is  to 
be  kept  at  its  lower  state  of  oxidation,  namely  the  chromic  state. 

From  the  above  it  is  clear  why  there  are  added  difficulties  in 
the  case  of  iron  tannage.  It  is  necessary,  first  of  all,  to  convert 
the  ferrous  iron  to  the  ferric  state  completely,  then  to  keep  it  in 
this  ferric  state  throughout  the  tanning  operation  under  adverse 
conditions,  and  to  prevent  the  separation  of  any  hydrated  ferric 
oxide,  Fe2O3.xH2O,  by  adjusting  the  proper  acidity  in  the  liquor 
employed. 


IRON  TANNAGE:  5 

SECTION  II.    HISTORY  OF  IRON  TANNAGE,  INCLUDING 
RECENT  WORK. 

Attempts  to  use  iron  salts  as  tanning  agents  date  back  to  the 
time  when  efforts  were  first  made  to  find  a  substitute  in  the  form 
of  metallic  salts  for  vegetable  tannins.  In  the  course  of  more 
than  a  century,  efforts  were  repeatedly  made  and  interest  was 
continually  revived  to  make  iron  tannage  a  commercial  success, 
but  without  reward,  in  spite  of  the  great  promise  that  inspired 
such  investigators  to  make  a  most  determined  effort.  Within  the 
last  decade,  partly  on  account  of  the  Great  War,  new  interest  has 
been  given  to  iron  tannage,  and  the  properties  and  behavior  of 
iron  salts  as  tanning  agents  are  gradually  becoming  better  under- 
stood. 

The  history  of  iron  tannage  begins  from  the  latter  part  of  the 
eighteenth  century.  Many  of  these  early  investigators  are  men 
who  also  helped  to  establish  the  present-day  chrome  tannage. 

In  the  year  1770,  J.  Johnson,3  an  Englishman,  patented  a  pro- 
cess of  tanning  using  ferrous  sulphate  with  an  acid  (sulphuric 
acid,  hydrochloric  acid  or  nitric  acid).  The  pelt  was  tanned  in 
three  operations  in  the  middle  of  which  a  vegetable  tannin  was 
used. 

In  1794,  Sam.  Ashton,4  another  Englishman,  recommended  the 
use  of  a  mixture  of  iron  oxide  and  sulphuric  acid,  calcined  iron 
ore  or  iron  ochre  with  pyrites,  copper  ore,  and  zinc  for  tanning. 
The  duration  of  tanning  was  given  as  from  five  to  seven  weeks. 
For  calfskins  some  alumina  was  also  added. 

In  1805,  Sigmond  Hermbstadt5  in  his  book  on  leather  tanning 
explained  that  solutions  of  metallic  salt  baths  had  similar  action 
on  the  pelt  as  the  oak  tannins.  Among  other  salts  he  mentioned 
the  red  iron  sulphate,  in  which  not  only  the  grain  of  the  pelt  was 
affected,  but  the  pelt  was  virtually  converted  into  leather  if  soaked 
in  it  for  some  time.  He  prepared  his  iron  tan  liquor  by  heating 
ferrous  sulphate  to  a  dry  yellow  substance  which  was  dissolved 

*Handbuch  der  Chromgerbung,  Josef  Jettmar,  p.   133    (1900)  ;   also 
Die  Chromgerbung,  J.  Borgmann,  p.  6  (1902). 
'Ibid. 
6  "Le  Tannage  au  Fer,"  Le  Culr,  Josef  Jettmar,  June  15,  1919 


6  I^ATHER   CHEMISTS   ASSOCIATION 

in  twenty  times  its  volume  of  boiling  water,  and  on  cooling,  the 
clear  yellow-red  solution  decanted  for  use.  He  also  treated  iron 
oxide  with  acetic  acid,  or  oxidized  ferrous  sulphate  with  concen- 
trated nitric  acid  or  with  a  mixture  of  concentrated  nitric  acid 
and  sulphuric  acid.  Sole  leather  as  well  as  upper  leather  could 
be  made  in  this  way  but  the  leather  obtained  was  detanned  in  con- 
tact with  water. 

In  1842,  d'Arcet,6  a  Frenchman,  tanned  the  hides  in  a  ferric 
sulphate  solution  but  the  sulphuric  acid  set  free  gradually  de- 
stroyed the  hides.  In  the  same  year,  Julius  Bordier,7  of  L,ondon, 
patented  a  process  (British  Patent  9,219,  1842)  of  oxidizing  fer- 
rous sulphate  with  nitric  acid  and  sulphuric  acid,  and  with  man- 
ganese dioxide  and  sulphuric  acid.  It  was  said  that  he  had  at- 
tained some  success. 

In  1853,  Hylten  Cavalin,8  employed  for  tanning,  liquor  contain- 
ing 10  pounds  of  dichromate  of  potash  and  20  pounds  of  alum  in 
180  pounds  of  water.  The  hides  were  tanned  in  this  liquor  for 
four  days,  and  were  next  placed  in  a  10  per  cent,  ferrous  sulphate 
solution  for  twelve  hours  with  frequent  stirring.  The  acidity  re- 
lation of  these  two  liquors  was  not  properly  adjusted  and  the  iron 
was  not  completely  oxidized.  The  leather  obtained  was  hard  and 
brittle. 

In  1855,  Rene  de  Kercado  Molac  and  Jean  Daniel  Friedel,9  both 
of  Strasbourg,  France,  patented  a  process  whereby  the  hides 
were  tanned  in  a  basic  ferric  sulphate  solution  which  was  later 
neutralized  with  metallic  oxides,  such  as  ferric  oxide,  alumina, 
and  zinc  oxide  to  remove  the  sulphuric  acid  liberated.  They  pre- 
pared the  liquor  with  ferrous  sulphate,  manganese  dioxide  and 
sulphuric  acid,  and  added  to  the  liquor  ferric  acetate  in  varying 
proportions.  A.  E.  L.  Bellford,  of  London,  patented  their  pro- 
cess in  England.  In  this  British  patent  (January  12,  1855)  it 
was  stated  that  the  leather  treated  by  other  mineral  processes  is 
"liable  to  tear  in  length  of  time  on  account  of  the  great  quantity 
of  acid  remaining  in  the  leather  corroding  the  animal  fibres." 

'"Leather  Industry,"  A.  M.  Villon,  trans,  by  F.  T.  Addyman,  p.  189 
(1901). 

T  See  also  "The  Arts  of  Tanning,"  Campbell  Morfit,  p.  396  (1852). 
'"The  Arts  of  Tanning,"  Campbell  Morfit,  pp.  397-401   (1852). 
'Compare  "Handbuch  der  Chromgerbung,"  Jettmar,  p.  137. 


IRON  TANNAGE;  7 

Dr.  Frederich  L,.  Knapp,  Professor  of  the  Polytechnic  School 
of  Brunswick,  Germany,  made  a  thorough,  scientific  investigation 
on  these  mineral  tannages  and  published  the  results  in  "Die  Natur 
und  das  Wesen  der  Gerberei  und  des  L,eders"  (Munich,  1858), 
and  also  in  an  article,  "tiber  Gerberei  und  L,eder,"  in  Dingler's 
Polytechnische  Journal,  Vol.  181,  p.  311  (1866).  He  made  a  satis- 
factory explanation  of  the  tanning  action.  He  had  in  mind  the 
possibility  of  reducing  the  length  of  time  needed  in  the  vegetable 
tanning  process  and  eliminating  the  costly  materials  such  as  egg- 
yolk  and  flour  used  in  alum  tannage.  He  recognized  the  plump- 
ing effect  upon  the  hides  by  the  acid  liberated  during  tanning  and 
the  stiff  and  brittle  character  of  the  leather  obtained.  He  recom- 
mended neutralizing  the  tan  liquor  during  the  progress  of  tanning 
with  sodium  carbonate  or  caustic  soda  and  pointed  out  the  advan- 
tages in  so  doing,  namely,  that  the  hides  were  more  richly  tanned, 
that  the  harmful  acid  effect  was  prevented,  and  that  a  neutral 
electrolyte  Nad  (in  FeCl3  liquor)  was  produced  in  the  tan  liquor. 
His  English  patent  (British  Patent  2,716,  1861,  through  John  H. 
Johnson)  covered  iron,  chrome,  manganese,  and  other  metallic 
salts  in  combination  with  fatty  acids  to  form  insoluble  metallic 
soaps  so  that  the  iron  in  the  pelt  might  not  be  washed  out.  He 
also  mentioned  the  use  of  similarly  insoluble  silicates  of  alumi- 
num and  alkaline  earths.  According  to  his  patent  (German  Pat- 
ent No.  444,  1877)  ne  prepared  his  liquor  by  adding  nitric  acid  in 
excess  to  oxidize  completely  a  boiling  ferrous  sulphate  solution 
when  brown  nitrogen  dioxide  (NO2)  fumes  were  seen.  After  all 
iron  was  oxidized  he  introduced  more  ferrous  sulphate  into  the 
resulting  solution  as  long  as  NO2  fumes  were  evolved.  The  liquor 
after  evaporation  becomes  a  varnish-like  liquid.  Judging  from 
this  description  his  liquor  must  have  been  too  alkaline  through 
the  loss  of  the  nitric  acid  by  boiling.  In  his  additional  patent 
(German  patent  No.  10,518,  1879)  he  used  instead  of  the  nitric 
acid,  sodium  nitrate  and  sulphuric  acid  for  oxidation.  This 
method  is  far  more  economical  and  involves  no  danger  of  losing 
the  acid  by  heating  so  that  the  acidity  of  the  resulting  liquor  is 
under  control.  Furthermore,  a  neutral  salt,  Na2SO4,  is  produced 
in  the  tan  liquor. 

In  1864,  F.  Pfannhauser10  obtained  a  patent  for  the  preparation 
""Manufacture  of  Leather,"  Chas.  T.  Davis,  p.  290  (1897). 


8  LEATHER   CHEMISTS   ASSOCIATION 

of  a  basic  ferric  sulphate  solution  and  its  use  in  tanning.  He 
roasted  ferric  sulphate  to  a  red  heat  with  continuous  stirring  until 
it  was  reduced  to  a  red  powder  which  was  then  thrown  into  water 
while  still  hot.  Most  of  this  powder  was  said  to  be  dissolved. 
The  suspension  was  allowed  to  settle  and  the  supernatant  liquid 
drawn  off  for  the  preparation  of  tan  liquors  of  varying  strength. 
The  skins  were  tanned  count-ercurrently  and,  when  tanned,  placed 
in  a  soap  solution. 

In  1877  Paesi11  proposed  to  use  a  ferric  chloride  solution  to- 
gether with  salt  at  20°  C.  in  the  ratio  of  100  parts  of  water  to 
ten  parts  of  FeCl3  and  five  parts  of  salt. 

In  1881,  E.  Harcke  obtained  a  German  patent,  No.  19,633, 
according  to  which  the  pelt  for  making  sole  leather  was  treated 
with  a  mixture  of  a  resinous  body  (such  as  rosin),  coal  tar  creo- 
sote, or  carbolic  acid,  and  an  alkali,  in  water,  until  thoroughly 
penetrated.  The  pelt  was  then  tanned,  first  in  an  aluminum  salt 
solution  and  then  in  a  ferric  chloride  solution,  or  other  ferric  salt 
solution.  For  making  upper  leather  the  hides  were  previously 
limed  and  if  softness  and  porosity  were  desired,  the  rosin  could 
be  omitted. 

In  1881,  W.  Eitner12  patented  a  process  (Austrian  Patent  No. 
6,775)  using  a  mixture  of  a  basic  chromic  sulphate  and  ferric 
sulphate  solution.  This  process  was  used  in  Graz,  Austria,  and 
the  product  known  as  "Patentleder,  Marke  Elefant."  By  chang- 
ing the  ratio  of  the  chromic  salt  to  the  ferric  salt  different  grada- 
tions of  color — from  yellow  (of  the  iron)  to  gray  (of  the  mix- 
ture) and  to  green  (of  the  pure  chrome) — were  obtained.  When 
a  mixture  of  the  ferric  and  chromic  salts  was  used,  the  leather 
was  colored  black  with  logwood  alone;  when  chromic  salt  alone 
was  used,  the  leather  was  colored  black  with  logwood  and  an 
iron  "striker."  When  yellow  color  was  not  desired  in  the  pro- 
duct, chromic  salt  alone  was  used  for  tanning.  Leather  obtained 
in  this  way  was  stuffed,  after  sammying,  with  mixtures  of  train 
oil,  castor  oil,  stearin,  tallow,  mineral  oil,  etc.,  with  soda  bicar- 
bonate, soap,  borax,  casein,  etc.,  as  emulsifying  agents. 

'"Leather  Industry,"  A.  M.  Villon,  trans,  by  Addyman,  p.  189  (1901). 

tt"Die  Chromgerbung,"  J.  Borgmann,  pp.  49-54  (1902);  also  "Hand- 
buch  der  Chromgerbung,"  Jettmar,  p.  151  (1900). 


IRON    TAN  NAGS  9 

In  1886,  John  W.  Fries,  of  Salem,  North  Carolina,  patented  a 
process  of  tanning  (U.  S,  Patents  Nos.  343,166  and  343,167) 
using  ferrous  carbonate  (or  ferrous  sulphate),  sodium  carbonate 
(or  sodium  bicarbonate)  and  sulphuric  acid.  The  skins  were 
tanned  first  in  a  dilute  liquor  for  two  or  three  days  and  then  in 
a  more  concentrated  liquor  for  the  same  length  of  time.  A  small 
amount  of  sugar  might  be  added.  After  the  tanning  operation 
the  skins  were  hung  in  the  air  to  get  the  iron  oxidized.  For 
currying,  he  used  tallow  with  a  paraffin  oil,  lard,  or  cotton  seed 
oil,  and  later,  in  his  patent  No.  343,167,  he  recommended  an  alco- 
holic solution  of  castor  oil. 

In  1892,  Paul  F.  Reinsch,  Erlangen,  Bavaria,  patented  a  pro- 
cess (German  Patent  No.  70,226)  using  a  liquor  prepared  by  mix- 
ing 10  kg.  FeCl3  dissolved  in  40  1.  of  water  with  4^  kg.  crystalline 
Na2CO3  dissolved  in  20  1.  of  water,  thus  yielding  a  dark  brown 
solution.  He  called  it  ferric  chloride-sodium  chloride  liquor, 
which  he  used  for  making  different  kinds  of  leather  either  alone 
or  in  combination  with  alum-sodium  chloride  tannage.  In  1912 
he  obtained  another  German  patent,  No.  265,914,  on  the  use  of 
ferric  chloride  and  magnesium  carbonate.  He  prepared  the  liquor 
by  dissolving  I  kg.  ferric  chloride  in  4  1.  of  water  to  which  was 
added  a  suspension  of  225  g.  MgCO3  in  a  liter  of  water.  To  this 
mixture  he  added  a  solution  of  8  per  cent,  aluminum  chloride. 
Evidently  his  idea  is  to  bring  about  the  required  basicity  by 
MgCO3.  The  A1C13  present  is  probably  meant  to  help  keep  the 
basic  ferric  chloride  in  solution. 

J.  Bystron  and  Karl  Baron  von  Vietinghoff  obtained  a  number 
of  German  patents,  Nos.  255,320,  et  seq.,  in  1911,  a  British  patent, 
No.  13,952  in  1912,  and  two  U.  S.  patents,  No.  1,048,294  in  1912 
and  No.  1,061,597  in  1913.  They  employ  nitrogen  dioxide,  NO2, 
and  nitrogen  trioxide,  N2O3,  for  the  oxidation  of  iron.  The  nitric 
oxide,  NO,  from  the  oxidation  reaction  is  collected  and  reoxidized 
by  contact  with  fresh  air  to  NO2  and  N2O3,  which  gases  are  used 
over  again  for  oxidation.  They  thus  proposed  to  utilize  the  NO2 — 
NO — NO2  cycle,  making  the  oxides  of  nitrogen  virtually  catalytic 
agents  for  the  oxidation  of  iron.  In  the  British  patent,  No.  13,952, 
they  observed  considerable  precipitates  formed  in  the  tan  liquor 
and  on  the  skin.  According  to  them,  the  presence  of  large  quan- 


IO  LEATHER   CHEMISTS   ASSOCIATION 

titles  of  an  acid  causes  the  formation  of  a  highly  acid  and  not 
completely  insoluble  iron  oxide  in  the  skin  so  that  the  leather 
made  is  brittle  and  can  not  be  stored.  In  this  patent  and  also  in 
the  U.  S.  patent  No.  1,048,294  he  proposed  placing  the  skin  in  a 
ferrous  salt  solution  and  oxidizing  the  ferrous  iron  by  passing  in 
NO2  gas  from  outside  or  by  liberating  HNO2  from  a  nitrite  added 
to  the  liquor.  Thus  they  attempt  to  combine  the  oxidation  reac- 
tion and  the  tanning  operation  in  a  single  procedure.  It  is  true 
that  HNO2  (from  a  nitrite  and  an  acid)  has  sufficiently  high  oxi- 
dation potential  to  oxidize  ferrous  iron  to  the  ferric  state,  but  in 
order  to  oxidize  all  the  ferrous  iron  into  the  ferric  state  com- 
pletely, the  presence  of  much  acid  in  the  solution  and  of  an  excess 
of  the  oxidizing  agent  is  needed.  If  the  oxidation  by  HNO2  or 
oxides  of  nitrogen  is  to  take  place  simultaneously  with  tanning 
operation  at  the  low  acidity  necessarily  present  in  the  tan  liquor, 
probably  there  will  be  much  difficulty  in  getting  all  of  the  ferrous 
iron  completely  oxidized.  Bystron  in  the  U.  S.  patent  No.  1,061,- 
597  patented  the  use  of  a  neutral  alkali  salt  such  as  Na2SO4  or 
NaCl  for  treating  the  iron-tanned  leather.  He  claimed  that  by 
this  treatment  a  more  insoluble  basic  ferric  salt  of  a  light  color 
is  formed  in  the  leather,  thereby  yielding  a  soft,  elastic,  and  non- 
brittle  leather. 

O.  Rohm  in  1917  obtained  British  patents  Nos.  103,827  and 
104,338  on  the  combination  tannage  using  formaldehyde  and 
ferric  chloride,  or  formaldehyde  and  a  mixture  of  ferric  chloride 
and  chromic  chloride  or  aluminum  chloride.  In  his  patent  No. 
I03»295  (n°t  accepted)  he  mentioned  the  use  of  ferric  alum  mixed 
with  vegetable  tannins  to  form  iron  tannate  (ink)  for  tanning. 
In  his  patent  No.  103,827  he  recommended  tanning  with  formal- 
dehyde in  sodium  bicarbonate  solution  followed  by  a  tannage  with 
a  ferric  chloride  solution,  a  mixture  of  ferric  chloride  and  chromic 
chloride,  a  mixture  of  ferric  chloride  and  aluminum  chloride,  a 
ferric  chloride  solution  and  then  vegetable  tannins,  or  a  ferric 
chloride  solution  with  an  alkaline  sulphide.  He  also  mentioned 
the  treatment  of  the  skin  with  an  iron  precipitant,  such  as  NH3, 
alkalies,  or  alkaline  salts ;  or  phenols,  naphthols,  organic  carbox- 
ylic  acids,  vegetable  tannins ;  or  soap,  sulphide,  polysulphide,  and , 
the  like.  He  mentioned  that  the  leather  obtained  would  not  be- 
come slippery  in  wet  condition  as  is  the  case  with  a  chrome 


IRON    TANNAGE  II 

leather.  In  his  patent  No.  104,338  he  stated  that  the  aldehyde 
tannage  could  be  advantageously  used  to  follow  iron  tannage  after 
neutralization  or  together  with  neutralization.  When  the  alde- 
hyde is  introduced  together  with  the  neutralization  after  the  iron 
tannage,  there  is,  according  to  his  observation,  an  advantage  that 
the  grain-drawing  so  common  in  a  mineral  tannage  will  be  pre- 
vented. His  thought  seems  to  be  along  the  line  that  since  alde- 
hyde tannage  is  carried  on  in  an  alkaline  solution,  the  introduc- 
tion of  the  aldehyde  tannage  after  the  iron  will  serve  also  as  a 
neutralization  operation  to  fix  the  iron  in  the  pelt.  We  have  tested 
this  combination  tannage  and  found  the  leather  so  obtained  satis- 
factory. But  since  formaldehyde  is  a  tanning  agent  by  itself,  to 
what  extent  the  iron  salt  has  contributed  to  the  tannage  is  diffi- 
cult to  tell. 

Emil  Kanet13  in  his  German  patent  No.  306,015  (1918)  intro- 
duced an  interesting  feature  in  the  mode  of  tannage.  He  derived 
the  tanning  action  by  the  hydrolysis  of  a  ferric  salt.  He  treated 
the  pelt  at  a  low  temperature  with  a  ferric  salt  solution  of  such 
a  basicity  that  it  would  be  unstable  at  the  ordinary  temperature, 
and,  after  allowing  the  liquor  to  penetrate  the  pelt,  raised  the  tem- 
perature to  bring  about  hydrolysis.  To  illustrate,  he  placed  the 
skins  in  a  basic  ferric  acetate  liquor  containing  from  J/£  to  2.y* 
per  cent.  Fe2O3,  preferably  with  the  addition  of  some  salt  or  other 
electrolyte  such  as  sodium  acetate.  After  the  skins  were  pene- 
trated by  the  tanning  liquor  he  transferred  them  to  a  fairly  con- 
centrated salt  solution  at  a  temperature  of  from  45°  to  60°  C.,  or 
exposed  them  to  heat  in  a  warm  chamber.  The  tanning  action 
was  completed  in  a  short  time  but  the  stock  was  further  laid 
aside  for  some  time  to  fix  the  iron.  The  acetic  acid  set  free 
under  the  influence  of  heat  can  be  recovered  from  the  skins  by 
pressure.  If  a  filling  material  such  as  flour  is  used  with  the  tan 
liquor,  it  is,  according  to  him,  fixed  in  the  leather  with  the  basic 
ferric  acetate.  Other  mineral  salts  such  as  chromic  salt  can  be 
mixed  with  the  iron.  The  advantage  claimed  is  that  at  a  low 
temperature  a  more  basic  ferric  salt  solution  can  be  used  and  that 
the  oxidizing  activity  of  the  ferric  iron  towards  the  skins  is 
lessened. 

13  Compare  also  "Le  Tannage  au  Fer,"  by  J.  Jettmar,  Le  Cuir,  July  I, 
1919. 


12  LEATHER   CHEMISTS   ASSOCIATION 

W.  Mensing  in  his  Swiss  patent  No.  75,775  in  1918,  recognized 
the  ease  with  which  ferric  salt  in  solution  is  decomposed  and 
mentioned  the  effect  of  ferrous  iron  upon  the  skin  when  the  fer- 
rous salt  is  present  in  the  tan  liquor.  He  recommended  the  use 
of  an  excess  of  an  oxidizing  agent  and  patented  the  use  of  a 
chlorate  (Na,  K,  or  Ba)  as  the  oxidizing  agent.  He  also  recom- 
mended a  preliminary  treatment  of  the  skin  with  borax  or  a  basic 
aluminum  or  chromic  salt  solution  for  the  use  of  a  slightly  more 
acid  or  neutral  ferric  liquor.  According  to  his  idea  the  tanned 
stock  should  not  be  washed  with  water  but  only  wrung  or  pressed 
to  get  rid  of  the  excess  of  the  tan  liquor.  On  drying,  the  stock 
is  oiled  with  a  mineral  oil,  paraffin  or  ceresin  and  then  washed. 
To  avoid  reaction  of  the  iron  in  the  pelt  with  vegetable  tannins 
he  recommended  fixing  the  iron  by  treating  the  leather  with  a 
slightly  alkaline  solution  before  vegetable  retanning.  He  advo- 
cated the  bleaching  of  the  leather  by  detanning  the  surface  layers 
by  means  of  a  reducing  agent  and  then  an  acid.  On  the  whole, 
his  patent  marks  a  better  understanding  of  the  properties  of  the 
iron  tan  liquor  and  the  process  of  iron  tannage. 

Vittorio  Casaburi,14  in  the  articles,  "Notes  on  the  Tannage  of 
Skins  with  Iron  Salts,"  published  the  results  from  a  series  of  his 
experiments,  using  a  basic  ferric  sulphate  solution  (  from  the  oxi- 
dation of  ferrous  sulphate  with  a  mixture  of  nitric  acid  and  sul- 
phuric acid),  a  solution  of  a  mixture  of  basic  ferric  chloride  and 
sulphate  (from  the  oxidation  of  ferrous  sulphate  by  nitric  acid 
and  hydrochloric  acid),  a  basic  ferric  chloride  solution,  and  a 
basic  ferric  acetate  solution.  He  employed  a  strength  of  iron 
liquor  containing  I  per  cent.  Fe2O3  of  the  weight  of  the  pelt  in 
a  little  over  four  times  the  weight  of  water  of  the  weight  of  the 
pelt.  According  to  him  7.88  per  cent,  of  Fe2O3  in  the  leather 
on  the  basis  of  the  dry  weight  is  sufficient  to  convert  the  pelt  into 
leather.  He  stated  that  he  had  started  with  a  tan  liquor  having 
such  a  basicity  as  to  correspond  to  the  formula  Fe2(SO4)2(OH)2, 
but  his  iron  and  basicity  determinations  in  the  liquor  showed  that 
the  liquor  he  used  was  more  acid  than  this,  the  basicity  of  his 
first  liquor  (basic  ferric  sulphate)  being  only  one-half  of  this 
value,  and  that  of  his  second  liquor  (a  mixture  of  basic  ferric 
sulphate  and  chloride)  less  than  a  half  of  this  value.  We  have 
"  Le  Cuir,  Aug.  i,  Sept.  I  and  Sept.  15,  1919. 


IRON   TANNAGE  13 

found  that  a  sulphate  liquor  having  so  high  a  basicity  as  to  cor- 
respond to  Fe2(SO4)2(OH)2  is  too  alkaline  for  use.  Throughout 
the  course  of  tanning  he  strengthened  the  liquor  with  fresh  por- 
tions of  the  strong  liquor.  He  drew  a  conclusion  that  the  final 
basicity  of  the  liquor  was  the  same  as  that  at  the  beginning  of 
tanning — a  conclusion  that  has  not  been  confirmed  by  our  experi- 
ments. 


14  BATHER   CHEMISTS   ASSOCIATION 

SECTION  III.    INVESTIGATION  OF  DIFFERENT  METHODS  OF  OXIDA- 
TION AND  SOME  METHODS  OF  PREPARATION  OF 
IRON  LIQUOR  FROM  COPPERAS. 

As  the  largest  and  cheapest  commercial  source  of  iron  salts  is 
in  the  form  of  the  ferrous  sulphate  or  "copperas,"  FeSO4-7H2O, 
this  particular  salt  of  iron  naturally  forms  the  starting  point  for 
the  preparation  of  the  tanning  solutions.  As  the  ferric  salts  are 
generally  more  expensive  than  the  corresponding  ferrous  salts, 
economy  demands  that  the  iron  tanning  liquor  shall  be  made  from 
the  ferrous  salt — "copperas"  in  particular — rather  than  directly 
from  a  ferric  salt  purchased  as  such.  Hence  it  is  clear  that  a 
proper  method  of  oxidation  is  essential  to  the  preparation  of  this 
tanning  liquor  and  constitutes  one  of  the  main  factors  in  the 
economic  aspects  of  iron  tannage.  Consequently  it  is  worth  while 
to  devote  some  attention  to  the  study  of  different  methods  of 
oxidation  and  of  the  value  of  different  oxidizing  agents  from  the 
tanning  point  of  view.  In  this  investigation,  mostly  qualitative, 
we  have  constantly  kept  in  view  three  points,  namely  (i)  the 
simplicity  of  the  method  by  which  oxidation  can  be  carried  out; 
(2)  the  character  (acidity,  etc.)  of  the  liquor  thus  obtained  in 
regard  to  the  convenience  for  use;  and  (3)  the  cheapness  of  the 
chemicals  employed.  In  the  following  there  is  given  a  brief 
summary  of  the  properties  and  behavior  towards  ferrous  sulphate 
solution  of  some  of  the  more  important  oxidizing  agents,  although 
the  study  includes  practically  all  of  the  ordinary  oxidizing  agents 
available. 

Sodium  Dichromate,  Na2Cr2O7.2H2O.— Oxidation  goes  on  in 
the  cold  and  to  completion  (as  tested  with  K3Fe(CN)6  solution) 
even  in  the  absence  of  any  acid  added.  There  is  a  tendency  for 
the  ferric  oxide,  Fe2O3.xH2O,  to  separate  out.  With  a  small 
amount  of  a  mineral  acid  added  no  precipitate  will  be  formed 
and  the  reaction  is  distinctly  accelerated  by  the  higher  hydrogen- 
ion  concentration. 

Cr207=  +  6Fe++  +  i4H+  **->  2Cr+++  +  6Fe+++  +  ;H2O. 

This  method  is  an  important  one  and  embodies  one  mode  of 
tannage  found  to  give  satisfactory  results.  It  has  several  advan- 
tages: (i)  that  the  oxidation  reaction  requires  only  a  very  low 


IRON    TANNAGE  15 

hydrogen-ion  concentration,  so  that  the  acidity  of  the  liquor  ob- 
tained is  entirely  in  control;  (2)  that  the  oxidation  potential  is 
high  and  the  oxidation  reaction  is  completed  very  smoothly  in  the 
cold;  and  (3)  that  the  waste  product,  Cr+++  salts  left  in  the 
resulting  liquor  is  itself  a  valuable  tanning  agent  and  constitutes 
what  may  be  called  a  co-tanning  agent  with  the  iron.  Further- 
more, a  slight  excess  of  the  sodium  dichromate  in  the  liquor 
could  effectively  prevent  any  ferric  iron  from  being  reduced  to 
the  ferrous  state  in  the  course  of  tanning.  In  spite  of  the  present 
high  price  of  the  dichromate,  the  process  has  merits  of  its  own 
as  will  be  presented  in  detail  in  a  later  section. 

Sodium  Chlorate,  NaClO3. — The  oxidation  by  a  chlorate 
NaClO3  or  KC1O3  in  a, cold  solution  does  not  occur  without  the 
addition  of  a  mineral  acid  (HC1  or  H2SOJ.  On  adding  the  acid 
the  reaction  takes  place  rapidly  and  goes  to  completion  in  the  cold. 
The: solution  assumes  a  greenish-yellow  color,  probably  due  to 
some  chlorine  dioxide  formed,  C1O2. 

2NaClO3  +  HC1  *•-»  2NaCl  +  HC1O3. 

HC103  +  6FeS04  +  sHCl  ^>  2Fe2(SO4)3  +  2FeCl3  +  3H2O. 
3HC103  *^  H20  +  2C102  +  HC104. 

With  a  weaker  acid,  like  acetic  acid,  oxidation  takes  place  on 
heating,  giving  a  red  solution  due  to  the  formation  of  the  basic 
ferric  salt.  Without  any  acid  added  the  reaction  can  be  brought 
about  by  heating,  but  Fe2O3.xH2O  would  then  be  thrown  down. 
As  the  chlorate  is  rather  expensive,  especially  the  potassium  chlor- 
.ate,  the  process  will  not  be  economical,  although  W.  Mensing 
advocated  its  use  in  his  patent.15 

Manganese  Dioxide,  MnO2. — In  the  absence  of  any  acid,  no 
reaction  takes  place,  MnO2  being  insoluble.  On  adding  HC1, 
evolution  of  C12  gas  is  observed  and  the  solution  turns  yellowish, 
this  being  the  characteristic  color  of  the  ferric  chloride  in 
solution. 

MnO2  +  4HC1  *>*•>  MnCl2  +  C12. 
The  reaction  proceeds  to  completion  in  the  cold. 

If  in  place  of  HC1,  H2SO4  is  used,  the  yellow  color  does  not 
-develop.  The  reaction  goes  to  completion  only  when  a  large 

16  Swiss  Patent  No.  75,775,  Class  40,  February  i,  1918. 


l6  LEATHER   CHEMISTS   ASSOCIATION 

excess  of  MnO2  and  H2SO4  is  employed.  A  small  but  distinct 
amount  of  the  permanganate  is  formed  when  all  ferrous  iron  has 
been  oxidized.  Hence  if  MnO2  is  to  be  employed  as  the  oxidiz- 
ing agent,  HC1  rather  than  H2SO4  should  be  used.  Molac  and 
Friedel  in  1855  prepared  their  iron  liquor  from  ferrous  sulphate 
with  MnO2  and  H2SO4. 

Nitric  Acid,  HNO3.— i.  HNO3  alone.16  Dilute  HNO3  has 
scarcely  any  oxidizing  action  upon  a  dilute  FeSO4  solution  in 
the  cold.  With  somewhat  more  concentrated  HNO3  solution,  a 
black  coloration  gradually  develops  due  to  the  reduction  of  some 
HNO3  to  nitric  oxide,  NO,  which  unites  with  FeSO4  to  form 
the  ferrous  nitroso  compound  FeSO4.NO.  The  black  color  deep- 
ens on  warming  and  persists  even  on  boiling  if  the  concentration 
of  the  HNO3  in  the  solution  is  not  high  enough  to  effect  the  oxi- 
dation. With  the  addition  of  more  HNO3  or  with  an  increase 
in  the  concentration  of  the  HNO3  due  to  the  loss  of  water  by 
prolonged  boiling,  complete  oxidation  finally  takes  place  and  all 
of  the  nitroso  compound  is  decomposed.  The  solution  then  boils 
violently,  brown  fumes  of  nitrogen  dioxide,  NO2,  being  given 
off.  During  the  evolution  of  the  gas  the  temperature  of  the  solu- 
tion falls  4°  or  5°  C.  The  amount  of  HNO3  required  for  com- 
plete oxidation  depends  largely  upon  the  concentration  rather 
than  the  absolute  quantity  of  HNO3  present.  Starting  with  a 
saturated  solution  of  FeSO47H2O  (one  part  of  FeSO47H2O  in 
about  one  and  a  half  parts  of  water  by  weight)  and  using  1.42 
HNO3,  a  considerably  less  amount  of  the  HNO3  need  be  em- 
ployed, but  25  per  cent,  of  1.42  HNO3  of  the  weight  of  FeSO4.- 
7H2O  taken  is  found  to  be  the  working  minimum  under  such 
conditions.  Although  complete  oxidation  can  still  be  brought 
about  with  a  less  quantity,  say  20  per  cent.,  the  liquor  obtained 
becomes  too  alkaline  and  has  a  muddy  appearance,  due  to  the 
separation  of  the  ferric  oxide.  This  can  be  readily  understood 
when  we  see  that  when  the  ferrous  salt  is  oxidized  to  the  ferric 
salt,  the  solution  becomes  less  acid  and  the  greater  part  of  the 
HNO3  used  simply  goes  to  furnish  the  necessary  acidity  (see 
paragraph  under  HNO3  +  H2SO4  below).  Some  HNO3  is  lost 
by.  boiling. 

18  This  method  was  used  by  Knapp,  but  the  liquor  he  prepared  yielded 
much  Fe,O8.xH2O. 


IRON    TANNAGE  I? 

The  oxidation  of  the  ferrous  iron  to  the  ferric  state  by  HNO3 
in  the  cold  can  only  approach  completion  when  a  very  large  excess 
of  the  concentrated  HNO3  is  added  to  a  concentrated  ferrous  sul- 
phate solution.  Consequently  boiling  is  a  necessary  operation 
which  makes  the  process  less  simple,  as  boiling  nitric  acid  solu- 
tion requires  a  special  container  to  resist  corrosion. 

2.  HNO3  +  H2SO4.    As  was  said  above,  the  oxidation  of  the 
ferrous  solution  to  the  ferric  state  renders  the   solution  more 
alkaline,  so  that  an  acid  must  be  added  to  prevent  any  ferric  salt 
from  being  hydrolyzed  and  precipitated.     If  H2SO4  in  the  re- 
quired amount  is  added  to  supply  the  acidity,  the  HNO3  needed 
can  be  cut  down  from  25  per  cent,  of  the  weight  of  the  FeSO4.- 
7H2O  to  9.5  per  cent..    Thus — 

3FeS04.7H20  +  4HNO3***->3Fe(NO3)  (SO4)  +  NO  +  2H2O 
theoretically  requires  30.2  per  cent.  HNO3  of  FeSO4.7H2O. 
6FeSO4.7H2O  +  2HNO3  +  3H2SO4  **»-» 

3Fe2(S04)3  +  2NO  +  4H20 

theoretically  requires  only  7.55  per  cent.  HNO3  of  FeSO4.7H2O. 
As  HNO3  is  far  more  expensive  than  H2SO4  this  method  is  more 
economical. 

3.  H2SO4  +  NaN03.17—With  a  saturated  ferrous  sulphate  solu- 
tion (i  part  FeSO4-7H2O  in  about  IT/>  parts  of  water)  and  with 
25  per  cent,  of  H2SO4  and  NaNO3  each  in  excess  calculated  ac- 
cording to  the  following  reaction, 

6FeSO4.7H2O  +  4H2SO4  +  2NaNO3  ^-> 

3Fe2(S04)3  +  Na2S04  +  2NO  +  4H2O, 

the  reaction  goes  on  very  smoothly  by  continued  boiling.  The 
resulting  liquor,  thick  like  a  syrup  and  dark  red  in  color,  has  a 
specific  gravity  as  high  as  1.50 — 1.55  and  contains  iron  as 
Fe2(SO4)3  from  40-45  per  cent.  As  Chile  saltpetre,  NaNO3,  is 
much  cheaper  than  HNO3,  this  method  is  still  more  economical. 
Furthermore,  the  Na2SO4  formed  in  the  liquor  cuts  down  the 
amount  of  NaCl  needed  for  tanning.  Taking  4-5  per  cent,  as 
the  normal  figure  for  NaCl  used  on  the  weight  of  the  pelt,  this 
saves  about  25  per  cent,  of  NaCl  required. 

4.  HNO3  -)-  HCl,  aqua  regia. — The  reaction  starts  in  the  cold. 
When  only  a  small  quantity  is  added  to  the  ferrous  sulphate  solu- 

"This  method  was  used  by  Knapp,  but  the  details  differ. 


.l8  LEATHER   CHEMISTS   ASSOCIATION 

tion  the  black  color  of  the  nitroso  compound  is  observed,  but  on 
further  addition  of  aqua  regia  the  nitroso  compound  is  decom- 
posed and  the  reaction  goes  to  completion,  although  the  end- 
point  is  not  very  permanent.    The  solution  on  standing  gradually 
assumes  a  golden  yellow  color  due  to  some  hypochlorous  anhy- 
dride C12O  formed.    The  main  reactions  seem  to  be — 
HNO3  +  3HC1  —*  NOC1  +  C12  +  2H2O, 
NOC1  +  C12  +  3FeS04  «»  NO  +  FeCl3  +  Fe2(SO4)3, 
although  other  oxidizing  agents  such  as  HNO2  and  HC1O  are 
also  formed.    The  proportion  of  1.20  HC1  to  1.42  HNO3  is  3 — 3.5 
to  i  by  volume  of  the  concentrated  solutions. 

The  oxidizing  power  of  aqua  regia  seems  to  be  greater  than 
that  of  the  concentrated  HNO3  alone,  but  some  assert  that  there 
is  no  difference  in  oxidation  potential  between  aqua  regia  and 
concentrated  nitric  acid.18  As  aqua  regia  is  difficult  to  handle  and 
rapidly  corrodes  the  container,  this  method  of  oxidation  is  neither 
economical  nor  simple. 

It  will  be  noticed  that  with  the  possible  exception  of  HNO3  -f- 
HC1,  all  methods  involving  oxidation  by  HNO3  in  some  form 
require  a  boiling  temperature.  This  constitutes  a  very  unfortu- 
nate feature.  There  are,  however,  some  distinct  advantages  in 
the  case  of  NaNO3  +  H2SO4,  vis.,  (i)  that  the  materials  used  are 
cheap;  (2)  that  a  very  concentrated  liquor  can  be  obtained,  and 
(3)  that  with  a  proper  amount  of  H2SO4  employed  the  liquor 
obtained  is  stable  and  there  is  no  danger  of  deterioration  on 
storing. 

Chlorine  Gas,  C12. — The  oxidation  by  chlorine  is  very  smooth 
and  simple.  The  reaction  starts  in  the  cold  and  goes  to  completion 
when  the  gas  is  passed  in  under  a  small  pressure  and  when  effi- 
cient stirring  is  maintained.  The  reaction  furnishes  its  own 
acidity  and  in  the  right  proportion. 

C12  +  H2O  *»+  HC1  +  HC1O. 
HC10  +  FeS04  +  HC1  «**  FeCl.(SO4)  +  H2O. 
The  process  is  very  efficient  and  incurs  practically  no  loss  of  C12 
if  two  or  three  units  are  connected  in  series  and  the  solutions 
treated  countercurrently.    Iron  liquor  obtained  by  this  method  is 
"Moore,  "Aqua  Regia,"  /.  A.  C.  S.,  p.  1091  (1911),  and  "Aqua  Regia 
II,"  J.  A.  C.  S.,  p.  33  (1913). 


IRON    TANNAGE  19 

of  course  saturated  with  C12  and  so  contains  a  slight  excess  of  it, 
but  this  is  indeed  an  advantage  for  it  prevents  the  ferric  salt 
from  being  reduced  again  and  also  enables  the  liquor  to  be  kept 
in  storage  without  any  danger  of  deterioration,  i.  e.,  either  Fe2O3.- 
xH2O  separating  out  or  some  ferric  salt  changing  to  the  ferrous. 
As  liquid  chlorine  now  can  be  obtained  in  large  quantities  and  at 
a  reasonable  cost,  there  is  in  it  much  to  recommend  from  a  com- 
mercial standpoint. 

Bleaching  Powder,  CaCl .  CIO.— This  method  is  one  of  the  first 
used  by  us  in  this  research.  The  oxidation  goes  on  in  the  cold. 
With  large  excess  it  is  possible  to  oxidize  completely  the  ferrous 
iron  without  adding  any  acid,  in  which  case  a  precipitate  of 
Fe2O3.xH2O  is  liable  to  come  down.  With  a  weak  acid  present, 
e.  g.,  acetic  acid,  the  reaction  is  accelerated,  and  with  a  mineral 
acid  it  goes  to  completion  readily.  CaSO4.2H2O  is  thrown  down. 
Fe2O3.xH2O  is  more  readily  separated  from  this  liquor  probably 
due  to  the  greater  coagulating  influence  of  the  divalent  radicals. 
The  bleach  suspension  itself  reacts  alkaline  so  that  the  addition 
of  an  acid  is  rendered  more  necessary.  As  the  oxidizing  agent  is 
really  HC1O,  there  is  no  advantage  in  using  this  material  over 
liquid  chlorine  and  the  cost  is  greater  for  the  chlorine  content  in 
this  form.  The  bleach,  however,  can  be  used  to  advantage  in 
connection  with  iron  tanning  processes  (to  be  described  in  later 
Sections). 

Sodium  Nitrite,  NaNO2. — Unlike  NaNO3,  oxidation  begins  in 
the  cold,  but  basic  ferric  salt  would  be  precipitated  when  no  acid 
is  added.  On  adding  an  acid  (HC1  or  H2SO4)  the  red  precipi- 
tate redissolves  and  nitrogen  oxide  gases  are  rapidly  given  off. 

2HNO2  «H*  H2O  +  N2O3. 

N203  ?±  N02  +  NO. 

The  reaction  then  goes  to  completion  in  the  cold  when  an  excess 
of  NaNO2  is  present.  With  acetic  acid  the  reaction  can  also  go 
to  completion  giving  a  colloidal  suspension.  As  the  NaNO2  solu- 
tion reacts  alkaline,  the  addition  of  an  acid  is  all  the  more  neces- 
sary. The  oxidation  potential  of  HNO2  is  lower  than  that  of 
HNO3,19  although  the  latter  produces  no  appreciable  oxidation 
in  the  cold,  especially  when  the  solution  is  dilute.  Since  HNO2 
"Ihle,  Z.  f.  phys.  Chem.,  Vol.  19,  p.  577  (1896). 


2O  LEATHER   CHEMISTS   ASSOCIATION 

is  unstable  and  is  decomposed  readily,  the  loss  of  HNO2  through 
decomposition  and  volatilization  is  very  great.  Bystron  and  Viet- 
inghoff20  patented  a  cyclic  process  to  collect  these  gases,  oxidize 
them  all  to  NO2  by- air,  and  use  the  gas  again  for  oxidation.  It 
is  probable  that  such  a  method  can  not  be  carried  out  in  practice 
without  undue  complications. 

Hydrogen  Peroxide,  H2O2. — The  oxidation  starts  in  the  cold, 
but  does  not  go  to  completion  without  a  large  excess.  Even  with 
a  large  excess  the  end-point  is  not  stable.  Fe2O3.xH2O  is  thrown 
down  on  standing  for  a  few  minutes.  The  solution  is  red,  due 
to  the  basic  ferric  salt  formed.  If  an  acid  (HC1  or  H2SO4)  is 
added  the  color  of  the  solution  becomes  yellow  and  the  reaction 
goes  to  definite  completion,  giving  a  more  permanent  end-point. 

H,O2  +  2FeSO4  +  H2SO4  •»->  Fe2(SO4)3  +  2H2O. 
As  H2O2  cannot  be  obtained  cheaply,  at  least  at  present,  its  use 
will  not  be  commercially  practicable. 

Potassium  Per  chlorate,  KC1O4. — The  oxidation  starts  only  on 
warming  when  no  acid  is  added,  but  the  solution  then  becomes 
turbid.  On  adding  an  acid,  the  turbidity  clears  up  but  the  reac- 
tion does  not  go  to  completion  even  when  the  solution  is  heated 
to  boiling.  Thus  contrary  to  what  one  might  suppose,  the  oxida- 
tion potential  of  the  perchloric  acid  is  lower  than  that  of  the 
chloric  acid. 

Oxygen  From  the  Air. — The  oxidation  of  the  ferrous  sulphate 
by  air  oxygen  would  be  a  very  cheap  method  if  it  could  be  brought 
about  rapidly  enough.  Fine  bubbles  of  air  are  passed  through  the 
ferrous  sulphate  solution,  but  the  reaction  is  too  slow,  only  6  per 
cent,  being  oxidized  at  the  end  of  four  hours.  At  an  elevated 
temperature,  80° -90°  C.,  the  reaction  is  more  rapid,  but  even  then 
only  12  per  cent,  is  found  to  be  oxidized  in  four  hours.  10  per 
cent,  concentrated  H2SO4  of  the  weight  of  the  FeSO4.7H2O  in 
the  solution  (i  part  FeSO47H2O  to  2  parts  water)  should  be 
present,  otherwise  ferric  oxide  would  be  precipitated.  The  ordi- 
nary catalytic  agents,  such  as  the  mercuric  salt  (5  per  cent. 
HgCl2)  and  the  phosphate  (5  per  cent.  Na2HSO4)  do  not  seem 
to  help.  Ozonized  air,  or  air  led  through  an  electric  ozonizer, 
would  do  better,  but  would  be  expensive. 

/"German  Patent  No;  255,320   (1911)   and  a  number  of  patents  fol- 
lowing. 


IRON    TANNAGE  21 

Anodic  Oxidation. — Oxidation  by  electrolysis  seems  to  have 
some  possibilities.  The  anode  is  best  made  of  lead  but  the  cathode 
can  be  lead,  graphite,  copper,  or  even  iron.  These  materials  have 
been  tried.  Lead  and  graphite  as  cathodes  are  inert  in  the  acid 
(H2SO4)  solution  and  are  found  to  be  suitable.  Copper  cathode 
is  not  attacked  by  the  acid  when  the  cells  are  running,  but  when 
the  current  is  stopped  it  is  attacked  by  the  acid  with  the  aid  of 
atmospheric  oxygen,  thus  contaminating  the  electrolyte.  An  iron 
cathode  can  be  used  to  advantage  as  it  is  of  the  same  material  as 
the  liquor  is  composed  of.  As  the  cathode  environment  is  a 
reducing  one,  a  diaphragm  such  as  porous  earthware,  asbestos 
felt,  or  electro-filtrose  should  be  provided  to  separate  the  cathode 
portion  from  the  main  electrolyte.  This  arrangement  prevents 
the  hydrogen  gas  evolved  at  the  cathode  from  mixing  with  the 
main  electrolyte.  The  cathode  chamber  need  not  be  large,  and  a 
capacity  of  about  one-fifth  or  less  of  the  volume  of  the  main 
electrolyte  is  sufficient.  The  cathode  solution  can  best  be  a  plain 
H2SO4  solution  and  should  be  maintained  at  a  higher  level  than 
the  body  of  the  electrolyte  to  prevent  diffusion  of  much  iron  into 
the  chamber.  It  is  found  that  the  H2SO4  concentration  inside 
this  chamber  should  be  maintained  high,  about  two  or  three  times 
as  high  as  in  the  main  electrolyte,  otherwise  some  iron  might  be 
plated  on  the  cathode.  The  electrolyte  is  made  by  dissolving  cop- 
peras in  about  twice  its  weight  of  water  and  adding  15  per  cent, 
concentrated  H2SO4  of  the  weight  of  copperas  taken.  As,  gen- 
erally speaking,  the  oxygen  over-voltage  is  low21  oxygen  gas  is 
easily  caused  to  be  discharged  at  the  anode,  resulting  in  low  cur- 
rent efficiency.  To  prevent  this,  the  anode  current  density  must 
be  low,  i.  e.,  the  anode  surface  must  be  large.  It  is  found  that  with 
the  anode  current  density  of  0.20 — 0.40  amperes  per  square  deci- 
meter for  a  liquor  containing  25 — 40  per  cent,  of  copperas,  the 
over-all  current  efficiency  is  as  high  as  70 — 75  per  cent.,  even  when 
there  is  no  circulation  or  stirring  in  the  main  electrolyte.  With 
good  circulation  or  stirring  and  with  a  concentrated  electrolyte  (30 
— 40  per  cent.  FeSO4.7H2O)  a  higher  current  density  can  be 
safely  employed  without  any  danger  of  the  discharge  of  oxygen 
gas  at  the  anode.  The  cell  takes  a  voltage  of  2.4 — 3.0  volts  depend- 
ing upon  the  distance  between  the  electrodes,  the  condition  of  the 
21  See  Allmand,  "Applied  Electrochemistry,"  p.  144  (1920). 


22  LEATHER   CHEMISTS   ASSOCIATION 

diaphragm,  the  concentration  of  the  electrolyte  and  its  tempera- 
ture, but  with  the  cells  running  properly  and  with  the  electrodes 
about  4  inches  apart,  this  terminal  potential  drop  should  not  be 
much  over  2.6  volts  under  normal  conditions.  The  back  E.  M.  F. 
is  approximately  2  volts.  Oxidation  can  go  to  completion  by  this 
method,  but  the  end-point  is  not  quite  permanent.  As  each  cell  on 
the  average  takes  less  than  3.0  volts,  there  can  be  connected  in 
series  on  the  120  main  about  40.  cells.  In  this  way,  the  method 
compares  favorably  with  the  cheapest  chemical  methods.  The 
disadvantage  seems  to  be  that  considerable  amount  of  Fe2O3.xH2O 
is  thrown  down  as  sludge  in  the  cells  and  the  cells  need  close 
attention  and  regulation  in  regard  to  the  acidity  in  the  cathode 
chamber,  proper  conditions  of  the  diaphragm,  etc.,  otherwise  sec- 
ondary reactions  might  set  in  and  the  cells  fail  to  function  prop- 
erly. It  is  found  that  the  lead  anode  is  oxidized  only  after  all 
iron  has  been  oxidized. 

From  the  above  brief  description  it  will  be  seen  that,  to  pro- 
duce such  a  cheap  product  as  the  ferric  salt,  many  of  the  costly 
and  rarer  oxidizing  agents  will  find  no  place.  Considerations  of 
the  different  factors  point,  for  the  present  at  least,  to  the  methods 
oxidation  by  chlorine,  oxidation  by  NaNO3  and  H2SO4,  oxida- 
tion by  HNO3  and  H2SO4,  and  oxidation  by  Na2Cr2O7  utilizing 
the  chrome.  The  other  methods  that  possess  some  possibilities 
are  anodic  oxidation  and  oxidation  by  the  atmospheric  oxygen  in 
some  form,  while  oxidation  by  NaClO3  or  other  chlorate,  oxida- 
tion by  NaNO2,  and  oxidation  by  MnO2  and  HC1  seem  to  have  a 
doubtful  economic  value. 

The  details  of  a  few  methods  of  preparation  of  the  iron  liquor, 
which  have  been  found  suitable,  will  now  be  given.  They  are 
based  on  the  oxidation  by  (I)  liquid  chlorine,  (II)  sodium  nitrate 
and  sulphuric  acid,  (III)  nitric  acid  and  sulphuric  acid,  and  (IV) 
sodium  dichromate.  Liquid  chlorine,  as  far  as  we  know,  has 
never  been  used  before.  While  sodium  nitrate  and  nitric  acid 
used  for  oxidation  in  conjunction  with  sulphuric  acid  are  more  or 
less  well  known22  the  details  of  procedure  in  regard  to  the  propor- 
tions of  the  materials  employed,  the  concentration  aimed  at  and 
acidity  desired,  etc.,  are  worked  out  independently.  The  condi- 

23  H,SO4  and  HNO8  used  as  early  as  1842  by  Bordier,  and  H2SO4  and 
JNaNOs  in  1879  by  Knapp. 


IRON    TANNAGE  23 

tions  as  given  here  are  those  found  capable  of  producing  (i)  a 
high  concentration  of  iron  in  the  liquor,  (2)  complete  oxidation 
of  iron  with  some  excess  of  the  oxidizing  agent  in  the  liquor,  (3) 
complete  reaction  involving  the  use  of  a  minimum  amount  of  the 
oxidizing  agent  and  other  materials,  and  (4)  a  degree  of  acidity 
as  near  that  suitable  for  tanning  operation  as  possible,  consistent 
with  the  stability  of  the  liquor.  The  one  difficulty  with  the  pre- 
pared sulphate  liquor  is  that,  unless  the  degree  of  acidity  is  above 
a  certain  minimum  it  does  not  keep  well  and  ferric  oxide  is  liable 
to  separate  out.  The  separation  of  much  ferric  oxide  would 
greatly  impair  the  tan  liquor  and  this  danger  should  always  be 
guarded  against  when  the  liquor  is  to  be  placed  on  the  market 
where  its  keeping  quality  is  of  vital  importance. 

(I)  Oxidation  by  Chlorine. — For  laboratory  preparation.  A 
desired  weight  of  commercial  copperas  is  placed  in  1^4  to  iJ/£ 
times  its  weight  of  water  in  a  large  container  provided  with  an 
entrance  and  an  exit  hole.  Through  one  hole  is  passed  a  delivery 
tube  reaching  to  the  bottom  of  the  container.  During  the  passage 
of  the  gas,  the  contents  are  stirred  constantly.  As  the  copperas 
crystals  gradually  disappear  more  can  be  added  until  the  total 
weight  of  the  copperas  used  is  equal  to  the  weight  of  the  water 
present.  Toward  the  end,  the  exit  hole  is  stopped  temporarily  to 
create  a  small  pressure  of  the  chlorine  gas  above  the  solution. 
The  completion  of  oxidation  is  tested  with  K3Fe(CN)6  solution. 
The  end-point  should  be  so  permanent  that  a  test  sample  with 
K3Fe(CN)6  solution  should  be  colored  deep  red  and  remain  so 
for  at  least  one-half  hour  in  contact  with  air. 

To  the  liquor  35  per  cent,  commercial  NaCl  and  10  per  cent, 
soda  ash  of  the  weight  of  the  copperas  taken  are  added,  the  latter 
being  first  dissolved  in  a  small  quantity  of  water  and  added  very 
slowly  with  stirring.  This  liquor  thus  neutralized  should  be  used 
without  much  delay.  To  make  the  liquor  keep  for  a  short  period, 
pass  in  again  chlorine  gas  under  a  small  pressure  and  immediately 
stopper  the  bottle  tightly.  For  long  storage,  it  is  safer  not  to  add 
this  quantity  of  Na2CO3  until  ready  for  use.  The  bottle  should 
be  tightly  closed  so  that  no  chlorine  gas  can  escape.  No  acid 
need  be  added  to  the  ferrous  sulphate  solution  for  chlorine  oxida- 
tion. 


24  LEATHER   CHEMISTS   ASSOCIATION 

For  commercial  preparation,,  cast  iron  or  enameled  iron  tanks 
may  be  used.  Two  or  three  units  sould  be  connected  in  series 
and  the  gas  passed  in  countercurrently. 

The  liquor  thus  prepared  is  dark  red  in  color,  but  should  be 
absolutely  clear,  and  should  remain  so  without  depositing  yellow 
hydrated  ferric  oxide  on  standing.  It  is  rather  thick  and,  after 
the  addition  of  NaCl,  has  a  specific  gravity  of  1.39.  It  contains 
approximately  32  per  cent,  of  iron  calculated  as  Fe2(SO4)3. 

On  the  basis  of  100  pounds  of  the  drained  pelt,  the  cost  of 
preparation  is  estimated  as  follows : 

Copperas 14      pounds  at  i^      a  pound  $0.14 

Liquid  chlorine 2      pounds  at  7^      a  pound  .14 

NaCl  (crude) 5      pounds  at  y^    a  pound  .Q2l/2 

Na2CO3  (comm.) 1^/2  pounds  at  i^  a  pound 


Total    $0.32^ 

This  will  give  approximately  2^4  gallons  of  the  liquor  in  a 
concentrated  form,  weighing  about  31  pounds.  For  use,  dilute 
to  15 — 25  gallons  for  drum  tannage.  The  cost  of  preparation  per 
gallon  of  the  concentrated  liquor  is  estimated  to  be  about  12  cents, 
or  per  pound  a  little  over  i  cent. 

(II)  Oxidation  by  NaN03  and  H2SO±. — For  laboratory  prep- 
aration. Take  a  desired  quantity  of  commercial  copperas  and 
place  it  in  a  large  container  containing  about  i1/^  times  its  weight 
of  water  to  which  have  been  added  30  per  cent,  of  66°  Be.  H2SO4 
and  12^2  per  cent,  of  Chile  saltpetre  of  the  weight  of  the  cop- 
peras taken.  Heat  to  boiling  and  boil  gently  until  brown  fumes 
of  NO2  are  finally  given  off.  Remove  the  burner  during  evolu- 
tion of  the  gas. 

Add  20  per  cent.  NaCl  and  neutralize  slowly  with  9^2  per  cent, 
soda  ash  previously  dissolved  in  a  small  quantity  of  water.  The 
liquor  is  ready  for  immediate  use. 

For  Commercial  Preparation.  Use  an  enameled  open  kettle 
provided  with  a  steam  jacket  taking  exhaust  steam.  The  kettle 
should  have  a  somewhat  larger  capacity,  as  during  evolution  of 
the  gas  the  liquor  foams  badly. 

The  evolution  of  NO2  fumes  indicates  the  end-point  for  the 
reaction  and  that  a  small  excess  of  HNO3  is  present.  This  rep- 


IRON    TANNAGE  25 

resents,  of  course,  a  loss  in  HNO3  though  small,  which  would 
otherwise  be  available  for  oxidation.  But  as  a  small  excess  is 
always  necessary  to  carry  the  reaction  to  completion,  this  minor 
loss  seems  to  be  unavoidable. 

The  above  proportion  of  NaNO3  and  H2SO4  represents  an 
excess  of  15  —  20  per  cent,  over  the  theoretical  quantity  in  each. 
This  is  found  to  be  the  minimum  quantity,  especially  in  the  case 
of  NaNO3,  in  order  to  secure  a  complete  oxidation  without  ren- 
dering the  resulting  liquor  too  alkaline.  Rather  prolonged  boil- 
ing is  needed  before  NO2  fumes  are  giveri  off,  as  a  certain  con- 
centration of  HNO3  in  solution  must  be  attained  before  the  com- 
plete reaction  can  take  place.  But  the  liquor  obtained  is  more 
concentrated  due  to  the  loss  of  water  by  boiling. 

Before  the  addition  of  NaCl  and  Na2CO3  the  liquor  has  a 
specific  gravity  of  about  1.50  containing,  in  this  condition,  about 
36  per  cent.  Fe2(SO4)3.  It  is  a  thick  liquid,  dark  red  in  color. 
It  is  absolutely  clear  and  should  remain  so  on  long  standing  with- 
out deposition  of  ferric  oxide. 

On  the  basis  of  100  pounds,  the  cost  of  manufacture  is  esti- 
mated as  follows  : 

Copperas  ...............  14      pounds  at  i^      a  pound  $0.14 

H2SO4  (66°  Be.)  .......  4Vs  pounds  at  i$      a  pound  .O4Y3 

Chile  Saltpetre  .........   i^  pounds  at  3^      a  pound  .05^ 

NaCl  ...................  3      pounds  at  l/2$    a  pound  .01  1/2 

Soda  ash  ...............  iy2  pounds  at  ij^tf  a  pound 


Total    $0.2775 

This  gives  approximately  2.5  gallons  of  the  concentrated  liquor 
weighing  about  30  pounds  before  neutralization  with  soda  ash. 
For  use,  dilute  the  liquor  to  15  —  25  gallons  for  drum  tannage. 
The  cost  of  preparation  is  estimated  to  be  about  1  1  cents  per  gal- 
lon of  concentrated  liquor,  or  about  I  cent  per  pound. 

(Ill)  Oxidation  by  HN03and  H2SO4.—  For  Laboratory  Prep- 
aration. Take  a  desired  quantity  of  copperas  and  place  it  in  a 
container  containing  \y2  times  its  weight  of  water  to  which  have 
been  added  9^-10  per  cent,  of  1.42  HNO3  and  19  per  cent,  of 
66°  Be.  H2SO4.  Proceed  as  in  (II).  35  per  cent.  NaCl  of  the 
weight  of  the  copperas  taken  and  gl/2  per  cent,  soda  ash  are  later 
added. 


26  LEATHER   CHEMISTS   ASSOCIATION 

The  use  of  HNO3  alone  without  sulphuric  acid  is  very  expen- 
sive and  wasteful. 

The  liquor  has  the  same  general  properties  as  that  prepared  by 


On  the  basis  of  100  pounds  of  the  hide,  the  cost  of  manufacture 
is  as  follows : 

Copperas 14     pounds  at  itf      a  pound  $0.14 

HNO8  (1.42)   il/s  pounds  at  $4      a  pound        .202/3 

H2SO*  (66°  Be.) 22/3  pounds  at  i<fr      a  pound        .O22/3 

NaCl . 5      pounds  at  y2^    a  pound        .02^ 

Na»CO3 ilA  pounds  at  il/24  a  pound        .02 

Total    $0.3 1 5/e 

The  resulting  liquor  is  of  about  the  same  volume  as  that  in 
(II)  but  weighs  a  little  less.  The  cost  of  preparation  is  estimated 
to  be  about  13  cents  per  gallon  of  concentrated  liquor,  or  a  little 
over  i  cent  per  pound. 

(IV)  Chrome-Iron  Liquor. — For  Laboratory  Preparation.  Take 
a  desired  quantity  of  copperas  and  place  it  in  an  equal  weight  of 
water.  Add  35  per  cent,  of  66°  Be.  H2SO4.  Stir  until  as  much 
of  the  copperas  is  dissolved  as  possible.  Cool  and  add  gradually, 
with  stirring  and  cooling,  20  per  cent,  of  sodium  dichromate 
crystals  of  the  weight  of  the  copperas  taken,  and  then  add  30  per 
cent.  NaCl.  The  liquor  is  ready  for  use  without  neutralization, 
or  with  but  a  small  quantity  of  an  alkali  added. 

For  Commercial  Preparation.  The  proportions  and  procedure 
hold  good,  except  that  an  enameled  tank  or  crockery  should  be 
used  and  cooling  coils  of  hard  lead  should  be  provided. 

Considerable  heat  is  evolved  upon  the  addition  of  H2SO4  and 
in  the  introduction  of  Na2Cr2O7.2H2O,  so  that  cooling  facilities 
should  be  provided  in  commercial  work  when  a  concentrated 
liquor  is  desired.  When  the  liquor  is  made  by  the  tanners  them- 
selves for  immediate  use,  the  following  procedure  can  be  adopted  : 
For  each  100  pounds  of  the  drained  skins,  take  10  pounds  of  cop- 
peras in  40  pounds  or  5  gallons  of  water,  contained  in  crockery 
ware.  Add  3^5  pounds  of  66°  Be.  H2SO4,  stir  well  and  cool, 
and  then  gradually  introduce  2  pounds  of  Na2Cr2O7.2H2O  with 
good  stirring.  Dilute  to  15 — 25  gallons  for  drum  tannage.  No 
alkali  need  be  added. 


IRON    TANNAGE  2/ 

The  liquor  is  rather  thick  and  appears  black,  as  the  red  color 
of  the  iron  and  the  green  of  the  chromic  salt  tend  to  neutralize 
each  other.  After  the  addition  of  NaCl,  it  has  a  specific  gravity 
of  1.49  and  contains  about  26  per  cent,  iron  calculated  as  Fe2- 
(SO4)3  and  8^2  per  cent,  chromium  as  Cr2(SO4)3. 

On  the  basis  of  100  pounds  of  drained  pickled  sheepskins,  the 
cost  of  preparation  is  estimated  as  follows : 


Copperas  

.  .10 

pounds 

at    i^ 

•& 

pound 

$0.10 

H2SO4  (66°  Be  ) 

'B  pounds 

at    i^ 

•} 

pound 

Na3Cr2O7  2HaO 

.    2 

pounds 

at  28^ 

•a 

pound 

.56 

NaCl  

.    7 

pounds 

at  y*$ 

fl 

pound 

.015^ 

Total    $0.70%, 

The  liquor  obtained  in  this  concentrated  form  measures  approx- 
imately 2.2  gallons  and  weighs  27^/2  pounds.  The  cost  of  prepar- 
ation is  about  32  cents  a  gallon,  or  2.6  cents  a  pound. 

This  liquor  is  different  from  any  of  the  above  in  that  it  con- 
tains a  basic  chromic  salt  as  well  as  ferric  salt,  which  chromic 
salt  also  contributes  to  the  tannage.  While  it  costs  two  and  a 
half  times  as  much  as  the  liquor  obtained  by  any  of  the  first  three 
methods,  its  cost  is  still  very  much  less  than  that  of  a  one-bath 
chrome  liquor. 


28  LEATHER   CHEMISTS   ASSOCIATION 

SECTION  IV.     HYDROLYSIS  AND  DECOMPOSITION  OF  FERRIC  AND 

CHROMIC  SALTS  COMPARED. 

It  may  well  be  suspected  that  one  of  the  reasons  why  iron  liquor 
is  so  much  more  difficult  to  manage  than  chrome  liquor  might  be 
found  in  the  greater  tendency  of  the  ferric  salts,  particularly  the 
sulphate,  in  solution  to  yield  readily  a  precipitate.  On  the  other 
hand,  it  may  be  appreciated  why  chrome  liquor  is  advantageous 
when  the  peculiar  properties  of  the  chromic  salts  are  recalled. 
Chromic  salts,  both  the  sulphate  and  the  chloride,  in  solution  are 
capable  of  forming  "complexes."  The  bluish  hexahydrate  chro- 
mic chloride,  CrCl3.6H2O,  obtained  from  crystallization  in  the 
cold,  gives  a  violet  solution  which  turns  green  on  boiling.  This 
saturated  violet  solution,  when  treated  with  hydrogen  chloride  gas 
in  the  heat  yields  green  crystals,  the  aqueous  solution  of  which, 
when  freshly  prepared,  possesses  only  two  chlorine  atoms  out  of 
the  three  in  the  molecule  that  are  precipitable  by  AgNO3.23  Chro- 
mic sulphate  in  solution  also  forms  "complexes."  The  reddish- 
violet  crystals,  Cr2(SO4)3.i5H2O,  gives  a  bluish  violet  solution  in 
cold  water  which  also  turns  green  on  boiling.  This  green  com- 
pound can  yield,  with  H2SO4  on  warming,  products  whose  SO4- 
radical  is  not  precipitable  by  BaCl2.24  The  chemistry  of  chromic 
salts  in  solution  is  certainly  a  complex  one.  Certain  irregularities 
in  the  behavior  of-  a  chrome  liquor  may  possibly  be  due  to  this 
complex  nature.  As  far  as  is  known,  no  such  peculiarities  exist 
in  the  ferric  salt  solution. 

A  qualitative  comparison  between  the  behavior  of  a  neutral 
ferric  salt  solution  and  a  neutral  chromic  salt  solution  both  in  an 
excessively  diluted  form  is  instructive.  I  cc.  of  a  10  per  cent, 
neutral  ferric  sulphate  solution  is  diluted  to  100  cc.  and  allowed 
to  stand  for  24 — 36  hours.  A  fine  precipitate  of  yellow  hydrated 
ferric  oxide  copiously  settles  at  the  bottom.  With  further  dilu- 
tion to  300  cc.  and  standing,  about  two-thirds  of  the  original 
amount  of  iron  in  solution  separates  out  and  the  solution  becomes 
so  depleted  that  it  appears  almost  colorless.  When  diluted  to 
1,000  cc.  and  allowed  to  stand  for  three  to  four  weeks,  the  solu- 
tion gives  only  a  slight  blue  coloration  with  K4Fe(CN)6.  If  a 

23  See  Ostwald,  "Principles  of  Inorganic  Chemistry,"  translated  by  A. 
Findlay,  p.  636  (1914). 

"Ibid.    Also  Holleman,  "A  Text  Book  of  Inorganic  Chemistry,"  trans- 
lated by  H.  C.  Cooper,  p.  461  (1916). 


IRON    TANNAGE  29 

normal  chromic  sulphate  is  dissolved  and  diluted  to  the  same  de- 
gree, only  a  slight  turbidity  is  observed,  but  very  little,  if  any, 
precipitate  separates  out  after  24  hours'  standing  and  the  solution 
remains  light  green.  This  comparison  shows  in  a  qualitative  way 
that  the  chromic  salt  in  solution  is  very  much  less  susceptible  of 
hydrolysis  and  decomposition  with  dilution. 

Next,  a  mixture  of  a  basic  ferric  sulphate  and  basic  chromic 
sulphate  solution  was  quantitatively  studied.  This  solution  is 
prepared  by  placing  50  g.  of  ferrous  sulphate  crystals  in  50  cc. 
of  distilled  water  containing  17.4  g.  C.  P.,  1.84  H2SO4  and  then 
gradually  introducing  9  g.  of  sodium  dichromate  crystals,  Na2- 
Cr2O7.2H2O  to  the  mixture.  Upon  introduction  of  the  sodium 
dichromate,  ferrous  sulphate  crystals  are  dissolved,  much  heat  is 
given  off,  and  a  thick  dark  liquid  cpntaining  a  mixture  of  the  basic 
ferric  sulphate  and  chromic  sulphate  in  solution  is  obtained.  This 
solution  appears  black  by  reflected  light  and  dark  red  by  trans- 
mitted light.  It  has  a  specific  gravity  of  1.46.  Analysis  shows 
that  it  contains,  per  10  cc. — 

0.07312  equivalents  of  SO4-  (from  acidity  determination). 

0.06820  equivalents  of  Fe+++. 

0.02117  equivalents  of  Cr+  ++. 

The  ratio  of  the  number  of  equivalents  of  the  sulphate  radical 
SO4-  (which  is  divalent)  to  that  of  Fe  and  Cr  combined  (each  of 
which  is  trivalent)  is  0.819  :  i.ooo,  so  that  there  are  not  enough 
sulphate  radicals  to  go  with  iron  and  chromium  in  the  solution. 
This  condition  may  be  summarized  in  the  formula : 

Radical M111  :  SO4=  :    (OH~)  (M111  =  Fe+++  +  Cr+++) 

Ratio  of  equiv.  i.ooo:  0.819  :  0.181  (by  difference). 
The  idea  of  using  such  a  quantity  of  H2SO4  and  making  a  liquor 
as  concentrated  as  this  needs  some  explanation.  In  the  first  place, 
if  a  liquor  is  to  be  placed  on  the  market  it  has  to  be  in  as  concen- 
trated a  form  as  practicable  in  order  to  save  freight  charge  in 
transportation  and  to  avoid  inconvenience  in  handling.  In  the 
second  place,  a  tanning  liquor  must  always  contain  some  degree 
of  basicity.  Too  much  acid  left  in  the  liquor  not  only  means  just 
so  much  alkali  needed  for  neutralization  before  the  tanning  oper- 
ation, but  also  involves  a  danger  of  not  getting  the  proper  degree 
of  basicity  for  tanning  in  the  hands  of  men  who  are  not  quite 


30  LEATHER   CHEMISTS   ASSOCIATION 

familiar  with  chemistry.  We  find  by  a  number  of  experiments 
that  this  proportion  of  the  sulphuric  acid  represents  the  minimum 
quantity  that  should  be  present  in  order  to  make  the  liquor 
alkaline  enough  to  be  used  for  tanning  without  neutralization 
(Chrome-Iron  Joint  Tannage)  and  yet  acid  enough  to  make  the 
liquor  keep  for  a  considerable  period  without  danger  of  precipi- 
tation. 

A  quantity  of  this  concentrated  liquor  measured  through  a  bur- 
ette is  diluted  to  different  volumes  with  distilled  water,  and  the 
solution  allowed  to  stand  for  24 — 48  hours.  The  supernatant 
liquid  is  filtered  and  pipetted  for  analysis.  Chromium  is  deter- 
mined by  Na2O2  oxidation  and  by  titration  against  sodium  thio- 
sulphate  solution  using  KI  and  starch  as  indicator.  Ferric  hy- 
droxide that  is  precipitated  by  Na2O2  in  the  same  sample  is  fil- 
tered off,*  washed  and  dissolved  from  the  filter  with  hot  dilute 
HCl  solution.  The  amount  of  iron  in  this  solution  is  determined 
by  the  Zimmermann-Reinhardt  method.  These  results  are  tab- 
ulated as  follows : 

*When  much  iron  is  present,  a  considerable  amount  of  the  chromate 
is  absorbed  by  the  ferric  hydroxide  precipitated  from  the  peroxide  oxida- 
tion, so  that  the  result  of  the  chromium  determination  in  the  nitrate  is 
always  too  low  while  the  iron  by  the  Zimmermann-Reinhardt  method 
becomes  high.  To  avoid  this  error,  the  supernatant  chromate  solution  after 
the  peroxide  oxidation  is  decanted  through  a  filter  and  the  ferric  hydroxide 
left  in  the  beaker  is  dissolved  by  adding  a  small  amount  of  hot  dilute  HCl. 
The  solution  is  now  diluted  and  the  ferric  hydroxide  re-precipitated  with 
an  alkali  at  a  boiling  temperature.  A  small  amount  of  Na2O2  may  be 
introduced  with  the  alkali,  in  which  case  care  must  be  taken  to  decompose 
all  the  peroxide  again.  If  a  large  quantity  of  chromium  is  also  present, 
a  second  re-precipitation  is  necessary  in  order  to  remove  all  the  chromium 
from  the  ferric  hydroxide. 


IRON    TANNAGE 


*   2 

to   o 
o  o 


if 

*o  0*0 
22" 

H 

I       '3       I        S          5 

*^CC2  be 

0                               0                          0                               0                                     0 

111 

-«-                                  00                              CT>                                   0)                                           TJ- 

III! 

*§"£ 

«                          oo'                                00                                      00 

Cc.  KMnO4sol. 
consumed 
N  =  o  1139 
(for  iron) 

<&               £>            oT               §8                  tf 

Cc.  supernatant 
solution  taken 
for  analysis 

^s*-^?      -i—         ".—            "—              pl— 

o.^y-.S^          *j              ort-                  o'^                      e^ 

Character  o  f 
solution  after  2 
days  standing 

**o                  8<0          S&f-"         xfc^'S«8          Sc^cO 

w«             .2  .  a       *2«o»g     wg-g^j        J;*'^-0 

!j|    iill  Jlltl  sillll  illlii 

a.               a            0               0                  j 

Color  of  solu- 
tion after  di- 
lution 

3_            ^S^-    •2;5a          s«               =3^ 

S|        l^f  •sis      l&s        •S*'-. 

li      &%  §^.   II'B       §i;-| 

|l             lUll^I       PS             111! 

ii-                 cn              75                 cw                    </! 

o 

j| 

0 

n 
o 

^                        •  -                    to                       to 
T3                                                                        N                             5 

Sample 

•3°          2°             -2°                =2 

5rt        Ss       g-        *-•          °- 

•-1             yl  8"      j  °  S>          d  2  8             d  I  § 

£*         2s"     ^N       S             ~~" 

32  LEATHER   CHEMISTS   ASSOCIATION 

It  is  evident  that  the  extent  of  hydrolysis  increases  with  dilution. 


6HO  ;F±  6OH 


6H+ 


2Fe(OH)3  3H2S04 

1 

Fe2O3.xH2O  (yellow  crystalline  ppt.  of 
dehydrated  ferric  hy- 
droxide). 

This  is  strictly  according  to  the  Mass  Action  principle,  for  a 
greater  dilution  means  a  greater  active  mass  of  water  and  hence 
the  following  reaction  is  pushed  to  the  right  : 

Fe2(S04)3  +  6H20  ^±  2Fe(OH)3  +  3H2SO4. 
Another  way  of  interpreting  this  is  that  with  greater  dilution  the 
hydrogen  ion  concentration  is  correspondingly  lowered.     That  is 
to  say,  the  alkalinity  of  the  solution  is  increased  and  consequently 
the  degree  of  hydrolysis  is  increased. 

The  above  results  show  how  ferric  sulphate  in  solution  is  more 
readily  hydrolyzed  and  decomposed  than  the  chromic  sulphate.  To 
show  this  more  clearly  the  following  curves  are  plotted  : 


Curves  Showing  Fe2  (SOJ3  and  Cr2 
Remaining  in  Solution  its,  Dilution 


/  2 

Log.  of  Number  of  Times  of  Dilution 


IRON  TANNAGE; 


33 


Ratio  of  Fe203  to  Cr2O3  In  Solution 


Log.  of  Number  of  Times  of  Dilution 

The  precision  of  the  above  determinations  is  not  better  than  l/2—i 
per  cent.  For,  when  the  precipitate,  especially  in  the  last  two  solu- 
tions, separates  out  abundantly  in  a  fluffy  manner  and  only  the 
supernatant  clear  liquid  is  taken  for  analysis,  the  original  volume 
ratio  does  not  exactly  hold,  but  the  error  is  small  and  can  be 
neglected. 

A  mixture  of  the  corresponding  chlorides,  namely  the  ferric 
chloride  and  the  chromic  chloride,  was  taken  and  similarly  studied. 
The  basicity  relation  as  determined  by  analysis  was  as  follows: 

Radical M'"  :  Cl~     :     OH~    (M111  =  Fe+++  +  Cr+++) 

Ratioof  equiv.  i.ioo:  0.800  :    0.200     (by  difference). 

Dilution  in  much  the  same  way  as  in  the  corresponding  sulphate 
mixture  was  carried  out.  In  no  case  was  there  any  precipitate 
observed,  not  even  where  the  original  solution  of  the  mixture  was 
diluted  to  2,000  times,  the  original  solution  having  a  concentra- 
tion of  34.52  g.  iron  as  Fe2O3  and  20.00  g.  chromium  as  Cr2O3  per 
liter.  This  shows  remarkably  that  ferric  chloride  is  far  more 
stable  toward  dilution  than  is  ferric  sulphate  for  the  same 
basicity,  and  in  this  respect  ferric  chloride  behaves  in  much  the 


34  LEATHER  CHEMISTS   ASSOCIATION 

same  way  as  chromic  chloride  or  other  chromic  salts.  The  con- 
clusion to  be  drawn  from  this  would  be  that  in  order  to  get  a 
stable  liquor  as  much  of  the  ferric  salt  in  the  tan  liquor  as  possible 
should  be  in  the  form  of  a  chloride,  but  unfortunately  the  main 
supply  of  ferrous  salt  is  already  in  the  form  of  a  sulphate  ("cop- 
peras") owing  to  the  cheapness  and  convenience  of  sulphuric  acid 
for  pickling  purposes  in  the  foundry  and  steel  works.  With  the 
chlorine  oxidation,  however,  a  third  of  the  acid  radical  is  con- 
veniently secured  in  the  form  of  a  chloride. 


IRON    TANNAGE  35 

SECTION  V.  ON  THE  RELATION  OF  BASICITY  TO  STABILITY  IN 
.  IRON  LJQUOR. 

The  instability  of  an  iron  liquor,  or  rather,  the  ease  with  which 
hydrated  ferric  oxide  separates  out  from  a  solution,  depends 
upon  the  degree  of  acidity  of  the  solution.  The  liquor  used  in 
tanning  is  normally  more  alkaline  than  that  which  corresponds  to 
a  neutral  salt,  e.  g,,  Fe2(SO4)3.  The  liquor,  however,  reacts  acid 
even  when  it  is  constitutionally  basic.  If  an  alkali  is  introduced, 
the  OH~  ions  from  the  alkali  tend  to  precipitate  ferric  iron  as 
ferric  hydroxide  or  as  some  basic  ferric  compound,  but  the  super- 
natant solution  still  reacts  acid.  Only  after  all  the  ferric  iron 
has  been  precipitated,  does  the  solution  begin  to  react  alkaline. 

It  is  evident  that  in  order  to  study  the  stability  of  an  iron 
liquor  with  regard  to  its  basicity,  it  is  necessary  to  know  quanti- 
tatively the  relation  between  the  Fe+++  ion  in  solution  and  the 
acid  radical  or  radicals  present.  The  subject  presents  some  diffi- 
culty, as  the  acid  radical  in  ferric  sulphate  solution  may  be  com- 
posed of,  besides  the  sulphate  ion  SO4=,  such  other  negative  ions 
as  NO3~,  Cl~,  etc.  This  is  not  uncommon  as  the  ferric  sulphate 
in  commerce  is  generally  obtained  by  the  oxidation  of  copperas 
with  HNO3  and  H2SO4,  some  of  the  HNO3  may  remain  in  the 
ferric  salt  solution  formed.  In  order  to  eliminate  as  much  as 
possible  complexities  of  this  nature,  there  was  chosen  as  the  start- 
ing point  a  white  powder  of  ferric  sulphate  as  nearly  chemically 
pure  as  possible.  A  solution  of  this  salt  (which  dissolves  very 
slowly  in  water)  was  made  containing  133  g.  of  the  air-dried 
powder  to  a  liter.  The  solution  was  allowed  to  stand  in  a  closed 
bottle  for  four  weeks,  when  a  small  amount  of  precipitate  col- 
lected at  the  bottom.  The  solution  tested  for  Cl~  with  AgNO3 
solution  gave  a  negative  result.  It  was  then  tested  for  NO3~  by 
adding  concentrated  H2SO4  and  then  ferrous  ammonium  sulphate 
solution.  No  colored  ring  was  observed.  This  test  is  not  very 
delicate.  With  diphenylamine  in  H2SO4  solution  a  violet  to  blue 
coloration  is  observed,  but  as  the  solution  contains  ferric  iron 
this  test  for  NO3~  in  the  presence  of  iron  can  not  be  regarded  as 
conclusive.25  The  test  was  therefore  further  elaborated  by  dis- 
tilling with  ferrous  sulphate  and  H2SO4  and  receiving  the  dis- 
tillate with  a  50  cc.  3  per  cent.  NaOH  solution  contained  in  a  250 
25  See  Tradwell-Hall,  Analytical  Chemistry,  Vol.  I,  p.  394  (1916). 


36  BATHER   CHEMISTS   ASSOCIATION 

cc.  Erlenmeyer  flask.  The  distillate  was  acidified  and  shaken  with 
5  cc.  of  chloroform  after  adding  '5  cc.  of  10  per  cent.  KI  solu- 
tion.26 The  test  gave  a  negative  result  for  NO3.  After  obtaining 
conclusive  negative  tests  both  for  Cl~  and  NO3-,  it  was  then  neces- 
sary to  determine  the  quantitative  relationships  between  the  ferric 
iron  and  the  sulphate  radical  in  the  solution.  For  this  purpose  a 
50  cc.  portion  of  this  ferric  sulphate  solution  was  diluted  to  500 
cc.,  25  cc.  of  which  were  taken  for  each  of  the  analyses  described. 
In  order  to  establish  the  quantitative  relationships  accurately  it 
was  considered  advisable  to  determine  the  iron  and  the  acid  rad- 
ical (SO4=)  each  by  two  independent  methods.  Iron  was  deter- 
mined gravimetrically  by  precipitating  with  NH4OH  in  the  pres- 
ence of  NH4C1,  and  independently  again  by  the  Zimmermann- 
Reinhardt  volumetric  method.  The  SO4=  radical  was  determined 
gravimetrically  by  precipitating  with  BaCl2  in  HC1  solution  and 
independently  again  by  the  acidity  determination  by  titrating  in 
the  heat  against  N/io  NaOH,  using  I  cc.  J^  per  cent,  phenol- 
phthalein  solution  as  an  indicator.  In  each  analysis  at  least  two 
portions  were  carried  and  the  results  checked.  The  results  were 
tabulated  as  follows : 

TABLE  II.— RESULTS  OF  ANALYSES  FOR  Fe+++  AND  SO*- 
BY  INDEPENDENT  METHODS. 

No.  of  equivalents  con- 
tained in  the  25  cc. 
Method  of  determination  (dil.  solution) 

Fe+  +  +  Zimmermann-Reinhardt  0.003853 

Gravimetric  as  FeaO8  0.003866 

SCX—  Gravimetric  as  BaSCX  0.003861 

Titration  against  N/io  NaOH  0.003872 

NOTE.  I. — The  Zimmermann-Reinhardt  method  is  not  suited  for  HzSCX 
solution  because  (i)  the  ferric  sulphate  solution  in  the  presence  of 
tUSCX  has  a  less  pronounced  yellow  color  to  guide  the  reduction  by 
SnCl2,  (2)  the  reduction  by  SnCl2  is  much  slower  in  the  H^SCX  solu- 
tion than  in  the  HC1  solution,  and  (3)  in  the  H2$O4  solution  a  pre- 
cipitate is  more  liable  to  form  in  the  solution  during  the  reduction 
unless  a  large  excess  of  the  H2SO4  is  present.  HUPCX  alone  without 
MnSCX  and  H2SO4  was  used.  The  result  was  found  to  be  slightly 
affected  by  the  amount  of  excess  of  SnCl2  employed. 

NOTE  2. — In  the  acid  determination  by  titratipn  with  NaOH,  the  difference 
between  the  end  points  in  the  cold  and  in  the  heat  is  not  great,  being 
about  I  per  cent,  of  the  total  burette  reading  in  the  case  of  ferric 
sulphate  solution  and  iy2  to  2  per  cent,  in  the  case  of  ferric  chloride 
solution.  It  is  remarkable  to  note  that  the  corresponding  difference 

28  See  A.  A.  Noyes,  Quantitative  Chemical  Analysis,  (1915)  p.  113. 


IRON  TANNAGE;  37 

in  the  case  of  chromic  chloride  solution  is  as  much  as  10  per  cent,  of 
the  total  burette  reading. 

By  "No.  of  Equivalents"  is  meant  the  number  of  equivalent 
weights  of  Fe+++  and  SO4=  contained  in  the  above  25  cc.  of 
diluted  ferric  sulphate  solution,  so  that  if  the  solid  ferric  sulphate 
from  which  the  solution  was  made  is  chemically  pure,  i.  e.,  con- 
tains nothing  but  Fe2(SO4)3  and  water,  the  number  of  equivalent 
weights  of  Fe+++  (which  represents  3  equivalents  per  formal 
weight)  should  be  exactly  equal  to  that  of  SO4=  (which  repre- 
sents 2  equivalents  per  formal  weight).  The  ratios  of  the  number 
of  equivalent  weights  of  Fe+++  and  SO4=  in  the  solution  as  de- 
termined are  as  follows : 

TABLE  III. — RATIOS  OF  EQUIVALENTS  OF  Fe++  +  TO  EQUIVALENTS  OF 
SO4=  IN  FERRIC  SULPHATE  SOLUTION. 

SO4~  by  gravimetric    SO4  ~  by  titration 

Fe++  +  by  Zimmermann-Reinhardt  i.ooo  :  1.002  i.ooo  :  1.005 

Fe+  +  +  by  Gravimetric  i.ooo  :  0.998  i.ooo  :  1.002 

The  closeness  with  which  the  above  results  agree  indicates  that 
the  ferric  sulphate  employed  is  substantially  chemically  pure. 
Knowing  the  exact  constitution  of  the  ferric  salt  solution  it  was 
then  necessary  to  study  the  stability  of  the  ferric  salt  in  solution 
by  changing  the  degree  of  basicity.  This  was  done  by  adding  a 
calculated  amount  of  Na2CO3  solution  of  a  known  strength  to  a 
given  quantity  of  Fe2($OJ3  solution  and  finally  making  up  to  the 
same  total  volume  with  distilled  water  in  each  case.  In  this  study 
there  was  employed  a  strength  of  the  ferric  salt  solution  in  each 
case  not  far  from  that  of  the  iron  liquor  used  in  the  actual  drum 
tanning  operation.  This  was  estimated  to  be  from  2  to  4  per 
cent,  iron  calculated  as  Fe2(SO4)3.  In  this  connection  it  might 
be  added  that  Na2CO3  was -used  rather  than  NaOH,  as  in  actual 
practice  in  the  tannery  sal  soda  or  sodium  bicarbonate  is  generally 
used  for  such  a  purpose.  A  0.5000  N  Na2CO3  solution  was  made 
from  a  thoroughly  dried,  anhydrous  C.  P.  sodium  carbonate.  For 
study,  a  25  cc.  portion  of  the  above  ferric  sulphate  solution  con- 
taining 10.26  g.  Fe2(SO4)3  per  100  cc.  was  taken,  a  calculated 
quantity  of  this  Na2CO3  solution  run  in  from  a  burette,  and  the 
total  volume  made  up  to  65  cc.  with  distilled  water.  The  results 
were  tabulated  in  Table  IV.  Although  during  the  introduction  of 


38  LEATHER   CHEMISTS   ASSOCIATION 

Na2CO3  there  was  a  brisk  evolution  of  CO2  gas,  there  remained 
in  the  suspension  some  CO3=  ions  in  addition  to  the  OH"  ions, 
so  that  the  precipitate  formed  was  in  the  nature  of  a  basic  ferric 
carbonate  or  a  mixture  of  ferric  hydroxide  and  ferric  carbonate.* 
It  was  noted  that  the  precipitate  separated  out  from  No.  7 
more  readily  than  it  did  from  Nos.  8,  9,  10,  and  n.  It  is  clear 
that  the  ferric  sulphate  solution  having  a  basicity  higher  than  that 
which  corresponds  to  No.  6  is  unstable  and  a  yellow  precipitate  of 
Fe2O3.xH2O  or  some  basic  ferric  carbonate  soon  separates  out. 
It  was  found  from  the  tanning  experiments  also  that  this  degree 
of  basicity  in  the  case  of  a  sulphate  liquor  was  as  high  as  could 
safely  be  employed  without  danger  of  causing  a  yellow  precipitate 
to  separate  out  on  short  standing  and  also  that  if  the  iron  liquor 
employed  was  of  a  higher  basicity  than  this,  it  would  rapidly  im- 
part a  yellow  color  to  the  surface  of  the  pelt  after  drumming  from 
15  to  30  minutes. 

Because  of  the  purely  scientific  interest  involved  NaOH  was 
also  used  in  place  of  Na2CO3  so  that  in  this  case  only  OH~  ions 
were  introduced  and  the  complication  of  having  some  CO3=  ions 
present  in  the  solutions  was  eliminated.  The  results  are  shown  in 
Table  V.  Comparing  Table  IV  and  Table  V,  it  is  remarkable  to 
note  how  the  two  series  run  parallel  to  each  other.  Table  V  like 
Table  IV  also  indicates  that  in  the  case  of  the  sulphate  liquor  Nos. 
6  and  7  represent  the  highest  basicity  beyond  which  the  iron  liquor 
yields  much  precipitate.  The  difference  between  the  two  series 
is  that  in  the  case  of  Na2CO3  any  precipitate  first  formed  can  be 
caused  to  disappear  on  continued  shaking  to  a  thick  solution  from 
which  the  final  precipitate  appears  after  a  lapse  of  from  12  to  30 
minutes  thereafter  while  the  precipitate  from  the  NaOH  solu- 
tions is  immediate  and  persistent..  The  color  of  the  solutions  in 
Table  V  is  also  somewhat  deeper.  The  report  that  the  best  iron 
liquor  to  be  employed  for  tanning  is  that  which  possesses  a  basicity 
corresponding  to  the  formula  Fe(OH)SO4  is  not  borne  out  by 
this  study  or  by  the  tanning  experiments. 

There    are    other    evidences    that    independently    show    that 

*  To  determine  how  much  CO3—  is  present  in  the  suspension,  CO2  was 
distilled  from  rUafter  the  precipitate  had  just  separated  out,  with  excess 
of  HaSCX  and  received  in  a  known  NaOH  solution.  Titration  gave  the 
amount  of  CO3=  present  in  the  suspension  to  be  63.6  per  cent,  of  the  total 
added. 


IRON    TANNAGE  39 

Fe(OH)SO4,  or  a  basic  ferric  sulphate  corresponding  to  this 
basicity,  is  very  unstable  and  rapidly  yields  a  precipitate  from  its 
solution.  For  instance,  when  a  ferrous  sulphate  solution  is  oxi- 
dized by  hydrogen  peroxide  without  the  addition  of  any  acid,  a 
precipitate  is  soon  formed  in  the  solution.  This  basic  ferric  sul- 
phate is  formed  by  the  following  reaction : 

2FeSO4  +  H2O2  •»-*  2Fe(OH)SO4. 

Other  neutral  oxidizing  agents,  such  as  KBrO3,  that  can  effect 
this  oxidation  without  the  addition  of  an  acid  to  the  neutral  fer- 
rous sulphate  solution,  produce  similar  precipitates  in  the  result- 
ing ferric  salt  solution. 

In  the  case  of  ferric  chloride  solution,  the  difference  is  aston- 
ishing. A  similar  series  of  the  ferric  chloride  solution  was 
studied  using  Na2CO3  solution.  The  results  are  tabulated  in 
Table  VI.  It  is  remarkable  that  throughout  the  whole  series 
studied  no  precipitate  was  formed  after  one  week's  standing  even 
where  the  basicity  ratio  was  higher  than  the  highest  in  the  sulphate 
series  studied  above.  This  shows  indeed  that  the  ferric  chloride 
is  far  more  stable  in  a  basic  solution  than  the  corresponding  sul- 
phate. After  two  weeks'  standing,  however,  precipitate  began  to 
appear  in  solutions  Nos.  3,  4,  5,  and  6.  It  is  interesting  to  note 
that  here  as  in  the  sulphate  series  above  (Tables  IV  and  V)  the 
precipitate  separated  out  more  readily  from  solutions  of  lower 
basicity  in  the  series  (Nos.  3,  4,  5,  and  6)  than  form  those  at  the 
end  having  high  basicities,  such  as  Nos.  10,  n,  or  12. 

The  "acidity"  determination,  according  to  the  method  of 
Thomas  and  Baldwin27  -for  the  solutions  of  various  basicity  in 
the  series  studied  above,  can  not  be  made  in  the  case  of  the  ferric 
salt  solution,  because  it  is  found  that  ferric  iron  is  rapidly  reduced 
by  the  hydrogen  in  the  presence  of  the  platinum  electrode,  and  no 
reading  can  be  obtained. 

The  determination  of  the  hydrogen  ion  concentration  ("acid- 
ity") in  iron  tan  liquor  by  a  study  of  the  rate  of  hydrolysis  of 
sucrose  has  also  a  similar  difficulty  as  one  of  the  products  of 
hydrolysis  (glucose)  has  a  reducing  action  on  the  ferric  iron  in 
the  hot  solution. 

""The  Acidity  of  Chrome  Liquors,"  by  A.  W.  Thomas  and  M.  E. 
Baldwin,  This  JOUR.,  May,  1918,  p.  192. 


40 


LEATHER   CHEMISTS   ASSOCIATION 


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IRON    TANNAGE  43 

SECTION  VI.    BEHAVIOR  OF  THE  PEI/T  TOWARDS 
IRON  TAN  LIQUOR. 

In  view  of  the  report28  that  iron  tan  liquor  has  the  same  acidity 
at  the  end  of  the  tanning  operation  as  it  had  at  the  beginning  and 
also  of  a  proposed  process  of  iron  tannage29  using  the  iron  tan 
liquor  over  and  over  again  in  a  cycle  without  mentioning  the 
necessity  of  readjusting  the  acidity 'of  the  liquor  it  was  thought 
advisable  to  study  this  matter.  Pickled  sheepskin  was  cut  into 
rectangular  pieces  of  about  4  inches  by  5  inches.  These  pieces 
were  placed  in  tepid  water  for  a  short  time  and,  when  softened, 
introduced  into  cold  water  containing  a  small  amount  of  salt  to 
prevent  plumping.  The  skin  was  then  carefully  neutralized  with 
Na2CO3  until  all  mineral  acid  was  removed,  using  methyl  orange 
as  an  indicator.  The  skin  was  rinsed  off  and  the  excess  water 
squeezed  out  so  that  it  was  roughly  in  the  same  condition  as 
pickled  skins  that  have  been  horsed  up  over  night.  The  reason 
for  neutralizing  the  skin  in  our  experiments  was  to  avoid  intro- 
ducing into  the  tan  liquor  an  indefinite  amount  of  the  mineral 
acid  present  in  the  pickled  skin.  The  tan  liquor  used  in  these 
experiments  was  a  basic  ferric  sulphate  or  chloride  solution,  hav- 
ing in  general  a  basicity  of  3  equivalents  of  mineral  acid  radical 
to  4  equivalents  of  Fe+++  and  containing  iron  from  nl/2  to  16^2 
g.  Fe2O3  per  liter.  The  volume  of  the  tan  liquor  in  cc.  equaled 
from  3  to  4  times  the  weight  in  grams  of  the  skin  with  excess 
water  pressed  out.  Tanning  was  carried  out  in  a  glass  jar  of  i^ 
liters  capacity  set  in  a  bottle  shaker  making  about  35  R.  P.  M. 
The  actual  weight  of  the  skin  in  these  experiments  was  from  150 
to  200  grams,  and  the  total  volume  of  the  tan  liquor  600  cc.  The 
amount  of  iron  used  (calculated  as  Fe2O3,  therefore,  varied  from 
4.0  to  6.0  per  cent,  of  the  weight  of  the  pelt  in  the  thoroughly 
drained  condition.  Both  Na2CO3  and  NaOH  were  employed  to 
bring  about  the  proper  basicity  in  the  -liquor  for  tanning.  In  the 
case  of  Na2CO3,  CO2  gas  continued  to  be  given  off  during  tan- 
ning. 

The  detailed  procedure  was  as  follows.  A  desired  amount  of 
the  stock  ferric  solution  of  known  concentration  and  acidity  was 

28  "Notes  sur  le  Tannage  aux  Sels  de  Fer,"  by  V.  Casaburi,  Le  Cuir, 
August  i,  1919. 

29  Bystron  and  Vietinghoff's  Patent  (German  Pats.  Nos.  255,320  et  seq.). 


44  LEATHER   CHEMISTS   ASSOCIATION 

taken,  and  a  calculated  quantity  of  NaOH  or  Na2CO3  solution  of 
known  strength  added  in  order  to  obtain  the  desired  basicity  for 
tanning.  The  solution  was  then  diluted  according  to  the  above 
volume  relation.  The  skin  was  immediately  placed  in  the  liquor 
and  the  container  shaken  in  the  bottle  shaker.  5  cc.  samples  were 
taken  for  analysis  at  an  interval  of  15  minutes  or  longer.  Iron 
was  determined  by  the  Zimmermann-Reinhardt  method  and  the 
acidity  (H2SO4  or  HC1)  by  NaOH  titration  using  i  cc.  y2  per 
cent,  phenolphthalein  solution  as  an  indicator.*  The  acid  deter- 
mination was  obtained  by  titration  with  N/io  NaOH  first  in  the 
cold;  and  after  the  end-point  has  been  reached,  the  solution  was 
brought  to  just  below  boiling  and  titration  continued  until  the  end 
point  was  again  reached.  The  difference  between  the  cold  and  the 
hot  end-points  was  only  0.15  to  0.35  cc.  N/io  NaOH  for  a  total 
burette  reading  of  10 — 30  cc.  Four  independent  experiments 
were  carried  out  in  the  case  of  the  ferric  sulphate  liquor,  but 
one  set  of  data  and  results  from  one  of  these  experiments  will 
be  given  here  which  may  be  considered  as  typical. 

FERRIC  SULPHATE  TAN  LIQUOR. 
Data: 

Sheepskin  (with  excess  water  pressed  out) 167  g. 

Ferric  sulphate  liquor — 

(a)  Volume  for  tanning 600  cc. 

(b)  Concentration  (calculated  as  Fe2O3) 16.33  'g.  per  1. 

(SO4=  Equiv.) 

(c)  Ratio  of > —   - 0.742 

(Fe+  +  +  Equiv.) 

Salt  (about  5^  per  cent.) '. 9  g. 

*  Mineral  acid  and  iron  can  be  determined  in  the  same  sample  by  first 
titrating  with  NaOH  solution  in  the  hot,  allowing  ferric  hydroxide  to 
collect  at  the  bottom,  filtering  off  the  precipitate,  dissolving  it  from  the 
filter  with  a  hot,  dilute  HC1,  and  then  determining  the  iron  in  solution  by 
the  Zimmermann-Reinhardt  method. 

The  results  are  tabulated  in  Table  VII.  The  skin  at  the  end 
of  each  experiment  was  well  tanned  save  for  the  neutralization 
operation  which  would  be  required  in  actual  practice. 


IRON   TANNAGE 


45 


TABLE  VII.— BEHAVIOR  o*  NEUTRAI,  PEW  TOWARDS  BASIC 
FERRIC  SULPHATE  LIQUOR. 


S04 


Fe 


Time  interval 
between  which 
Sample     samples  were 
No.               taken 

Cc.  NaOH      Equiv. 
in  the  hot      H2SO4 
N  =  0.1096      per  1. 

Equiv. 
Cc.  KMnO4        iron 
N=  0.1004        per  1. 

Ratio  of  Equiv. 
of 

SO4=toFe+" 

I* 

2O.72 

0-455 

IO.I7 

0.613 

742 

IOOO 

2 

15  min. 

17.88 

0.392 

8.63 

0.520 

754 

1000 

3 

15  min. 

16.80 

0.368 

7-51 

0.452 

815 

IOOO 

4 

15  min. 

16.13 

0-354 

7.21 

0-435 

814 

IOOO 

5 

15  min. 

15-86 

0.348 

6.80 

0.410 

849 

IOOO 

6 

15  min. 

15-54 

0.341 

6.61 

0.398 

858: 

IOOO 

7 

15  min. 

15-30 

0.336 

6.45 

0.388 

866 

IOOO 

8 

15  min. 

15-15 

0.332 

6.38 

0.385 

862 

IOOO 

9 

15  min. 

15.10 

0.331 

6-34 

0.382 

867 

IOOO 

10 

30  min. 

14.87 

0.326 

6.28 

0.378 

862 

IOOO 

ii 

24  hrs. 

Continuous 

shaking 

14.51 

0.318 

6.17 

0.372 

855  :  1000 

*NoTE. — The  sample  of  this  basic  ferric  sulphate  tan  liquor  yielded  a  pre- 
cipitate on  standing,  but  after  the  skin  was  placed  in  it  and  tanned, 
all  subsequent  samples  no  longer  yielded  any  precipitate.  During 
tanning,  the  color  of  the  liquor  became  lighter — from  deep  red  to 
yellow — and  the  skin,  on  the  other  hand,  was  gradually  colored  red. 

An  additional  experiment  was  carried  out  using  a  ferric  chlo- 
ride liquor  in  place  of  the  ferric  sulphate  liquor  used  above.  In 
the  following  are  tabulated  the  data  of  this  experiment. 


FERRIC  CHLORIDE  TAN  LIQUOR. 
Data: 

Sheepskin  (with  excess  water  pressed  out) 152  g. 

Ferric  chloride  liquor — 

(a)  Volume  for  tanning 600  cc. 

(b)  Concentration  (calculated  as  Fe2O3) 11.60  g.  per  1. 

(Cl-  Equiv.) 

(c)  Ratio  of  -  - 0.765 


(Fe+  +  +  Equiv.) 
Salt  (about  5  per  cent.) 


8g- 


The  results  are  tabulated  in  Table  VIII.  As  in  the  case  of  the 
experiments  using  ferric  sulphate  liquor  the  skin  was  also  well 
tanned  in  the  ferric  chloride  liquor  and  no  neutralization  was 
effected. 


46 


CHEMISTS   ASSOCIATION 


TABLE  VIII. — BEHAVIOR  OF  NEUTRAL  PELT  TOWARDS  BASIC 
FERRIC  CHLORIDE  LIQUOR. 

HCl  Fe+~i"  + 


lime  interval 
between  which 

Cc.  NaOH 

Equiv. 

t 

Equiv. 

Ratio  of  Equiv 
of 

Sample 

samples  were 

in  the  hot 

HCl 

Cc.  KMnO4 

iron 

4--t--f 

No. 

taken 

N  =  0.1104 

perl. 

N  =  0.1087 

per  1. 

Cl      to  I 

re 

I* 



15.00 

0-333 

6.66 

0.435 

765  : 

IOOO 

2 

15  min. 

I3.I6 

O.29I 

5-74 

0-374 

778: 

IOOO 

3 

15  min. 

12.44 

0.275 

5-31 

0.346 

795  : 

IOOO 

4 

15  min. 

12.07 

0.267 

5-07 

0.331 

806  : 

IOOO 

5 

15  min. 

11-95 

0.264 

4.76 

0.3II 

849  : 

IOOO 

6 

15  min. 

11.90 

0.263 

4-63 

0.302 

871  : 

IOOO 

7 

15  min. 

11.89 

0.263 

4-59 

0.300 

876  : 

IOOO 

8 

15  min. 

11.82 

O.26l 

4.61 

0.301 

868  : 

IOOO 

9 

15  min. 

11.80 

0.26l 

4-51 

0.294 

887: 

IOOO 

10 

24  hrs. 

Continuous 

shaking 

11.70 

0.259 

4-47 

0.292 

887: 

IOOO 

*NoTE. — None  of  these  samples  (including  sample  No.  i)  of  this  basic 
ferric  chloride  liquor  yielded  any  precipitate  on  standing.  The  colors 
of  these  samples  were  decreased  from  deep  red  (sample  No.  i)  to 
light  yellow  (sample  No.  10).  On  the  other  hand,  the  red  color  of 
the  skins  became  deepened  as  the  tanning  operation  progressed. 

From  these  results  the  following  important  conclusions  can  be 
drawn : 

(1)  Iron  is  taken  up  by  the  skin  very  rapidly  at  the  beginning 
of  the  tanning  operation,  and  from  30  to  40  per  cent,  of  the 
total  is  taken  up  by  the  pelt  before  neutralization. 

(2)  The  mineral  acid  (sulphuric  acid  or  hydrochloric  acid)  is 
also  taken  up  by  the  skin  in  a  similar  manner,  the  total  amount 
absorbed  by  the  neutral  pelt  being  in  general  about  20  to  30  per 
cent,  of  the  total.     (If  pickled  skins  are  not  neutralized  before 
tanning  as  in  actual  practice,  a  correspondingly  less  amount  of 
the  acid  will  be  taken  up  by  the  skin.     That  the  skin  absorbs 
the  sulphuric  or  hydrochloric  acid  from  the  liquor  is  corroborated 
in  actual  tanning  practice  by  the  fact  that  only  70 — 80  per  cent, 
of  the  theoretical  amount  of  alkali  is  required  to  effect  the  com- 
plete neutralization.    See  later  Section  on  Pure  Iron  Tannage.) 

(3)  Although  the  mineral  acid  is  also  taken  up  by  the  pelt,  it 
is  not  taken  up  in  the  same  proportion  as  the  iron  so  that  the 
liquor  is  more  acid  towards  the  end  of  the  tanning  operation  than 
at  the  beginning. 


IRON    TANNAGE  47 

(4)  The  curves  of  absorption  of  both  the  iron  and  the  acid  by 
the  pelt  with  respect  to  the  time  of  tanning  approach  some  con- 
stant horizontal  lines  asymptotically. 

(5)  The  tanning  reaction  is  practically  completed  within  one 
and  a  half  hours  of  drumming  and  the  neutralization  operation 
can  begin  after  i^  to  i^  hours  of  drumming,  it  being  neither 
necessary  nor  advisable  *to  allow  the  pelt  to  remain  in  the  liquor 
for  longer  than  i^  hours  before  neutralization  takes  place. 

(6)  Both  the  basic  ferric  sulphate  liquor  and  the  basic  ferric 
chloride  liquor  behave  alike  towards  the  pelt,  the  only  difference 
being  that  the   ferric   chloride   liquor   possesses   decidedly   less 
tendency  to  yield  the  precipitate  of  hydrated  ferric  oxide  than 
does  the  ferric  sulphate  liquor. 

Although  burette  readings  for  NaOH  are  expressed  in  four 
significant  figures,  the  precision  for  the  results  obtained  in  many 
cases  is  probably  not  much  better  than  i  per  cent.  For,  in  the 
first  place,  the  samples  taken  for  analysis  are  small  (only  5  cc.) 
and,  in  the  second  place,  the  organic  particles,  such  as  fatty  mat- 
ters, skin  fibers,  etc.,  present  in  the  tan  liquor  make  accurate 
sampling  rather  difficult.  Furthermore,  the  presence  of  grease 
causes  tiny  drops  of  the  liquor  to  adhere  to  the  walls  of  the 
pipette,  so  that  frequent  cleaning  by  means  of  a  cleaning  solution 
is  necessary.  Considerable  difficulty  is  also  experienced  due  to 
the  fact  that  the  presence  of  the  organic  matter  in  the  liquor  inter- 
feres with  the  determination  of  iron  by  -the  KMnO4  titration. 
This  difficulty  is  finally  overcome  by  oxidizing  all  the  organic 
matter  present  in  the  sample  with  KMnO4  in  the  presence  of  HC1 
until  a  purple  color  is  seen.  The  sample  is  then  heated,  reduced 
with  SnCl2,  and  titrated  with  KMnO4  solution  in  the  usual 
manner. 

It  was  observed  in  tanning  experiments  using  a  wooden  drum 
that  a  completely  oxidized  iron  tan  liquor  gave  a  copious  preci- 
pitate when  tested  with  K3Fe(CN)6  solution  at  the  end  of  the 
tanning  operation.  Evidently  the  tan  liquor  is  subject  to  reduc- 
tion by  the  skin,  the  woody  material  and  the  metallic  parts  of  the 
drum,  and  organic  impurities  that  may  collect  in  the  drum.  The 
amount  of  reduction  in  the  tan  liquor  by  the  skin  alone  has  been 
determined.  For  such  purpose,  the  skin  was  tanned  in  a  glass 
container  with  a  completely  oxidized  ferric  sulphate  liquor.  The 


48 


U5ATHER   CHEMISTS    ASSOCIATION 


f/c.  -J/f 

Curves  Showing  Absorption  ofFe  and  H2S04  during  Tanning 


Cj    o 


0       0:15     0-30     045     IM      1:15      1=30      M5     2-W     2-15     230  24:00 

Time  of  Tanning  in  Hours  and  Minutes 


Curve  Showing  Increase  in  Acid  Concentration  during  Tanning 
For  Ferric  Sulphate  Liquor 


0.9 


0.8 


3? 


i 


0.7 


0.6 


0.5 


0       0=15     0:30     045    MO      M5      1:30      145      2-W     2=15     2-30  2400 

Time  of  Tanning  in  Hours  and  Minutes 


IRON    TANNAGE 


Curves  Showing  Absorption  ofFe  and  HC1  during  Tanning 


Concentration  in  Equivalent  per  Liter 

rx  Cb.  J2>  <5>  J5>  J5>  ?: 
^  CI  ho  Co  -fc>  0,  C 

.;? 

'X^^^ 

>^^^ 

**•  •*•< 

»—  i    i 

re* 

<•+ 

•  —  < 

mmmrn 

i 

Acid  Concentration  (HC2) 

0       0-/5     0:30     0=45    IW     1-15     MO.     MS    2-00 

Time  of  Tanning  in  hours  and  Minutes 


Curve  Showing  Increase  in  Acid  Concentration 
in  Fe  CI3  Jan  Liquor  during  Tanning 


0--3Q      0:45      WO 

Time  of  Tanning  in  hours  and  Minutes 


2400 


50  LEATHER   CHEMISTS   ASSOCIATION 

skin  was  allowed  to  remain  in  the  liquor  with  continuous  shaking 
for  24  hours.  In  this  particular  experiment,  an  amount  of  iron 
containing  14^  per  cent.  Fe2(SO4)3  of  the  weight  of  the  drained 
skin,  dissolved  in  water  equal  to  3^2  times  the  weight  of  the  skin, 
was  used.  Ferric  iron  in  the  sample  taken  at  the  end  of  24  hours 
was  determined  by  means  of  a  titanous  sulphate,  Ti2(SO4)3  in 
H2SO4  solution  using  ^  cc.  normal  NH4CNS  solution  as  an 
indicator,  while  total  iron  was  determined  by  KMnO4  titration 
after  oxidizing  off  all  the  organic  impurities  in  the  sample  with 
KMnO4.  It  was  found  that  4.4  per  cent,  of  the  total  iron  in  the 
liquor  had  been  reduced  to  the  ferrous  state.  This  amount  of 
reduction  was  exclusively  due  to  the  skin.  In  actual  practice 
where  a  wooden  drum  is  used  with  whatever  dirt  that  may  collect 
in  it,  the  percentage  of  the  iron  reduced  must  be  greater. 


IRON  TANNAGE:  51 

SECTION  VII.    EXPERIMENTS  ON  TANNING  WITH 
FERRIC  HYDROXIDE  HYDROSOL. 

A  ferric  hydroxide  hydrosol  was  prepared  and  applied  to  tan- 
ning for  two  reasons :  ( I )  that  it  was  desired  to  study  the  tanning 
action  of  ferric  iron  completely  in  the  colloid  state,  and  (2)  that 
as  a  sol  requires  for  its  stability  only  the  small  amount. of  an 
electrolyte  that  is  retained  with  the  colloid,*  the  tan  liquor  can  be 
considered  to  be  practically  free  from  an  electrolyte  and  conse- 
quently the  function  of  a  neutral  electrolyte  such  as  NaCl, 
Na2SO4,  etc.,  used  in  a  mineral  tanning  liquor  will  thereby  be 
disclosed.  The  sol  was  made  by  the  peptization  method.  For 
such  a  purpose  a  fairly  concentrated  (about  30  per  cent.)  ferric 
chloride  solution  was  taken  and  a  dilute  ammonia  solution  ( i  vol. 
0.90  NH4OH  to  8  or  10  vol.  water)  added  drop  by  drop,  while  the 
solution  was  stirred  by  a  mechanical  stirrer.  On  adding  NH4OH 
the  yellow-red  solution  turned  brown  and  finally  almost  black  by 
reflected  light.  The  addition  of  NH3  was  continued  until  solid 
particles  of  ferric  hydroxide  were  re-dissolved  only  after  5  to  10 
minutes  continuous  stirring.  The  sol  was  dialyzed  in  collodion 
sacks  first  in  running  tap  water  over  night  and  then  in  distilled 
water  for  6  days,  changing  the  distilled  water  three  times  every 
24  hours.  The  sol  thus  obtained  was  rather  diluted  as  much  water 
had  entered  the  sacks  during  dialysis  by  virtue  of  the  osmotic 
pressure,  and  it  was  necessary  to  concentrate  it-  by  evaporation. 
After  concentration  it  was  filtered  through  cotton. 

This  sol  was  coagulated  by  an  electrolyte  such  as  NaCl  when  a 
certain  concentration  of  it  was  reached.  On  standing  the  coagu- 
lated ferric  hydroxide  settled  at  the  bottom,  leaving  the  super- 
natant solution  colorless.  When  it  was  coagulated  by  HC1,  added 
in  some  excess,  however,  the  precipitate,  on  standing,  re-dissolved 
giving  a  yellow  solution  of  ferric  chloride.  A  dilute  sol  prepared 
as  above  but  dialyzed  for  seven  days  showed  a  misty  beam  of  light 
under  an  ultramicroscope.  The  particles  are  so  small  that  they 
can  not  be  counted  and  their  motions  are  not  discernible  although 
a  distinct  mist  is  seen.  The  electrical  charge  of  the  sol  was  de- 
termined by  cataphoresis.  This  was  determined  in  the  ordinary 
U-tubes  using  KC1  solution  of  the  same  conductivity  as  the  sol  in 
*  The  "Complex"  Theory  of  Colloids. 


52  LEATHER   CHEMISTS   ASSOCIATION 

the  upper  portions  of  each  branch  in  which  the  electrode  was  im- 
mersed. The  sol  moved  toward  the  negative  electrode,  showing  it 
to  be  positively  charged,  as  would  have  been  expected.  . 

The  tanning  action  of  the  sol  toward  a  carefully  neutralized, 
nearly  salt-free  sheepskin  was  studied.  For  this  purpose,  a  piece 
of  pickled  sheepskin  (4%  in.  x  8^4  in.)  was  first  softened  in 
water  in  the  presence  of  a  small  amount  of  salt.  It  was  carefully 
neutralized  with  Na2CO3  solution  using  methyl  orange  as  an  in- 
dicator, and  the  salt  was  washed  out  from  the  skin  as  far  as  pos- 
sible (without  causing  undue  plumping)  when  only  a  small 
amount  of  AgCl  was  formed  in  the  wash  water  tested  with  an 
AgNO3  solution.  The  excess  water  was  pressed  out  from  the 
skin  and  its  weight  in  this  condition  was  51  g.  A  250  cc.  portion 
of  the  ferric  hydroxide  hydrosol  (analysis  giving  0.963  g.  Fe2O3 
per  100  cc.)  was  taken  and  the  neutralized,  nearly  salt-free  skin 
shaken  in  it  continuously  for  2^2  hours.  The  sol  was  not  coagu- 
lated but  the  skin  swelled  to  about  three  times  its  original  thick- 
ness, becoming  stiff  and  rubber-like.  It  was  not  tanned.  Five 
per  cent,  salt  (2.5  g.)  was  next  added  and  the  shaking  continued 
for  one  hour  longer.  The  skin  had  fallen  but  was  not  tanned  and 
the  inner  layer  was  not  even  penetrated.  The  sol  was  coagulated 
by  the  addition  of  the  sodium  chloride. 

The  experiment  was  repeated  in  exactly  the  same  manner,  us- 
ing, however,  a  well  pickled  skin  instead  of  the  de-pickled  and 
salt-free  skin.  'No  salt  was  added.  The  hydrosol  was  again 
coagulated  by  the  small  amount  of  the  H2SO4  and  NaCl  left  in 
the  pickled  skin,  yet  the  skin  swelled  badly.  The  cross  section  of 
the  skin  showed  red  lines  on  the  outside  edges  and  a  large  white 
untanned  band  in  between. 

From  this  it  appears  that  the  presence  of  an  electrolyte  such  as 
NaCl  or  Na2SO4  in  the  tan  liquor  is  to  prevent  plumping  so  that 
the  pelt  is  kept  "fallen"  in  the  liquor  so  as  to  be  readily  pene- 
trated by  the  tanning  agent.  It  must  be  observed  that  the  plump- 
ing of  the  pelt  in  the  hydrosol  is  not  caused  by  the  acid  liberated 
during  tanning,  for  there  is  only  a  negligible  amount  of  an  acid 
radical  that  can  be  retained  in  the  sol,  but  by  simple  inhibition  of 
water  by  the  gelatine  material  of  the  pelt  in  this  neutral  sol.  It 
appears  also  that  ferric  hydroxide  in  a  pure  sol  form  can  not  be 
employed  for  tanning  purposes;  for  without  a  sufficient  amount 


IRON    TANNAGE  53 

of  an  electrolyte,  the  pelt  would  be  caused  to  swell  and  not  be 
penetrated ;  but  if  sufficient  electrolyte  is  added  to  prevent  plump- 
ing, the  sol  is  coagulated  at  once  to  a  gel  which  likewise  would 
not  penetrate  the  pelt.  It  is  evident  that  a  pure  iron  sol  will  not 
act  as  a  tanning  agent ;  hence  the  theory  of  leather  formation  by 
colloidal  co-precipitation  alone  is  open  to  doubt,  and  the  reaction 
may  be  purely  chemical. 


54  LEATHER   CHEMISTS   ASSOCIATION 

y 

SECTION  VIII.  GENERAL  EXPERIMENTAL  WORK  ON  IRON 
TANNING. 

The  requirements  of  a  good  tannage  are  ( I )  that  the  pelt  shall 
be  converted  into  a  net  work  of  isolated  fibers  and  become  no 
longer  putrescible,  and  (2)  that  this  conversion  shall  be  irrever- 
sible, i.  e.,  the  leather  so  obtained  shall  not  be  readily  affected,  or 
reverted  to  the  raw  condition,  by  water  or  other  agency  (acid- 
alkali  treatments  excepted).  In  addition  it  might  be  mentioned 
that  the  leather  obtained  shall  keep  well,  i.  e.,  it  shall  not  spon- 
taneously deteriorate  on  storing  or  in  use  within  a  reasonable 
length  of  period.  Iron  tannage  can  yield  a  leather  that  will  ful- 
fill all  these  requirements  when  it  is  properly  carried  out. 

In  research  work  on  this  subject  many  difficulties  have  been 
encountered.  Since  no  detailed  data  on  the  subject  are  available 
in  the  existing  literature,  much  time  has  been  spent  in  determining 
conditions  for  a  successful  working.  A  ferrous  sulphate  solution 
was  employed  as  such  and  found  to  have  no  tanning  property.* 
Oxidation  of  the  iron  was  then  resorted  to,  but  the  question  arose 
as  to  what  oxidizing  agent  should  be  employed  and  in  what  man- 
ner the  oxidation  should  be  effected.  Sodium  nitrite  and  sul- 
phuric acid,  bleaching  powder  and  acetic  acid  or  sulphuric  acid, 
sodium  dichromate  and  sulphuric  acid  were  among  the  oxidizing 
agents  first  employed.  The  question  of  completeness  in  oxidation, 
the  relation  of  acidity  of  ferrous  sulphate  solution  to  oxidation, 
and  the  reduction  of  the  ferric  iron  in  the  liquor  by  organic  mate- 
rials in  contact  with  it  were  matters  that  were  gradually  brought 
to  light.  The  basicity  of  the  iron  liquor  used  for  tanning  exercised 

*  The  experiment  was  carried  out  as  follows : 

Skin  (sheepskin  pickled,  with  water  pressed  out) 40     g. 

Salt  (5  per  cent.) 2     g. 

'  Ferrous  sulphate  (FeSO^HkO)    (14  per  cent.) 5.6  g. 

Water  (3  times  the  weight  of  skin) 120     cc. 

The  solution  was  then  made  slightly  alkaline  up  to  the  point  of  the 
beginning  of  the  precipitation  of  the  green  Fe(OH)2.  The  skin  was 
shaken  in  it  for  il/2  hours  and  found  to  remain  soft  and  unaffected.  On 
drying  the  skin  so  treated  was  practically  in  the  same  condition  as  a  dry 
pickled  skin  with  dark  spots  and  areas  of  contracted  grain  shown  on  the 
grain  side.  It  was  affected  by  water  and  plumped  in  it. 


IRON    TANNAGE)  55 

an  unlooked-for  influence.**  For,  when  it  was  not  properly  ad- 
justed, either  no  satisfactory  tannage  was  obtained,  or  much  yel- 
low precipitate  was  deposited  so  that  the  pelt  was  difficult  to  tan 
through. 

After  some  experimentation  a  tannage  was  found  possible  and 
the  pelt  was  apparently  well  tanned  while  in  wet  condition.  On 
drying,  however,  the  pelt  shrunk  and  the  color  of  the  grain  was 
uneven.  Dark  wrinkled  spots  appeared  on  the  grain  and  often 
the  skin  on  drying  looked  horny  and  transparent.  The  leather 
was  stiff,  and  the  grain  brittle.f 

Since  a  comparatively  high  percentage  of  the  iron  salt  of  the 
weight  of  the-  pelt  is  needed,  this  necessitates  the  use  of  a  small 
amount  of  water  and  the  employment  of  a  somewhat  concentrated 
liquor  ("short  liquor")  in  an  attempt  to  cut  down  the  quantity  of 


*  *To  investigate  the  effect  of  the  basicity  of  the  tan  liquor  on  the  skin 
(neutralized)  tanned  in  it,  four  tan  liquors  with  different  degrees  of 
basicity  were  prepared  and  the  skins  tanned  in  them.  The  skins,  after 
being  shaken  in  these  liquors  for  15  minutes,  were  examined  and  the 
results  tabulated  as  follows: 


IX. 

Basicity  ratio  Color  of  skjn  Character  of 

Equiv.  Fe  +++  Color  of  shaken  for  skin  after 

Equiv.  SO4  =  tan  liquor  iS.min.  15  min.  shaking 

6/5  Deep  red  Reddish  straw      Skin  penetrated  by 

(Clear  sol.)  color  sol. 

5/4  Dark  red  Light  red  Skin  tanned  through 

(Clear  sol.) 
4/3  Dark  red  Red-yellow  Skin  tanned  through 

(Cloudy  sol.) 
3/2  Black  red  Brown-yellow       Pigment  crust  on 

Turbid  sus.  surface  layer 

Skin  barely  tanned 
through 

It  will  be  noticed  that  when  the  liquor  is  of  the  right  basicity  the  color 
of  the  skin  in  contact  with  it  should  be  more  red  than  yellow. 


flf,  however,  before  the  pelt  (skin)  was  permitted  to  dry  completely 
it  was  stretched  or  knee-slaked  while  still  in  a  sammied  condition,  the 
brittleness  of  the  grain  on  complete  drying  could  be  overcome.  This 
amounts  to  separating  the  fibers  by  mechanical  means  rather  than  from 
the  natural  result  of  tanning.  A  well  pickled  skin  could  be  made  soft  and 
flexible  as  if  tanned,  when  worked  in  the  same  way. 


56  IvEATHER   CHEMISTS   ASSOCIATION 

the  iron  salt  taken.*  The  phenomenon  of  "grain  drawing"  is 
then  apt  to  occur  if  the  liquor  is  not  carefully  introduced  and  the 
pelt  properly  prepared. 

Finally,  after  satisfactory  tannage  could  be  obtained  the  prob- 
lem of  coloring  presented  another  phase  of  difficulty.  The  yellow- 
red  color  of  the  leather  makes  dyeing  to  a  light  color  ("fancy 
color")  impossible.  Further,  the  iron  in  the  leather  is  active  so 
that  it  combines  with  many  substances  to  form  insoluble  com- 
pounds having  generally  an  objectionable  color.  Vegetable  re- 
tanning  is  limited  to  cases  where  a  gray  or  black  leather  is  desired. 
The  interference  of  iron  with  dyeing  by  means  of  basic  dyes  to 
a  color  other  than  black  constitutes  another  difficulty  on  account 
of  the  chemical  action  of  iron  on  many  a  vegetable- mordant  re- 
quired when  such  dyes  are  to  be  used.  On  the  other  hand,  in 
certain  cases  where  chemical  activity  of  iron  is  utilized  for  the 
coloring  of  the  leather  by  a  treatment  with  substances  capable  of 
producing  color  lakes  with  iron  (e.  g.,  with  K4Fe(CN)6  for  blue 
coloring)  the  leather  then  becomes  hard  and  brittle,  probably  due 
to  the  withdrawal  of  iron  from  the  fibers  for  the  formation  of 
the  inert  color  lakes.  Consequently  the  leather  is  to  a  greater  or 
less  extent  detanned.f  Furthermore,  the  color  of  the  leather  thus 
produced  is  not  fast  and  is  slowly  washed  out  unless  this  treat- 
ment is  immediately  followed  by  oiling  or  fat-liquoring.  On  the 
other  hand,  the  same  detanning  effect  also  results  when  an  attempt 
is  made  to  bleach  the  iron  tanned  leather  by  means  of  a  reducing 
agent  such  as  bisulphite  or  thiosulphate  followed  by  an  acid. 

Some  methods  that  have  been  employed  to  overcome  these  diffi- 
culties may  be  mentioned.  It  was  found  that  improper  tannage 
more  than  anything  else  was  responsible  for  the  brittleness  of 
the  leather.  When  a  pelt  is  not  uniformly  tanned  through,  due  to 
either  the  liquor  employed  being  too  alkaline  or  subsequent  neu- 

*  Similar  improvement  has  been  successfully  made  in  chrome  tannage. 
Thus,  from  a  private  communication,  a  process  of  two-bath  chrome  tannage 
for  skins  consisting  of  (i)  pickling  with  3  per  cent,  salt,  I  per  cent.  66° 
Be.  H2SO4  in  2  gallons  water  per  100  pounds,  drumming  for  20  minutes ; 
(2)  chroming  with  3  per  cent.  NazCrzOr^HaO,  i  per  cent.  66°  Be.  H2SO4 
in  2  gallons  water,  drumming  for  2  hours;  and  (3)  reducing  with  il/2  per 
cent.  66°  Be.  HzSCX  in  i  gallon  water  (added  first)  and  14^  per  cent, 
"hypo"  in  2  gallons  of  water,  drumming  for  2  hours,  has  been  successfully 
employed,  giving  a  very  soft,  light-colored  leather. 

f  If  the  neutralized  tanned  pelt  is  first  dried  to  "crust"  and  then  wetted 
back  for  this  treatment,  the  effect  of  detanning  is  less  marked. 


IRON    TANNAGE  57 

tralization  too  rapid,  the  outer  layer  (the  grain)  becomes  dense 
and  crusty,  while  the  inner  layer  remains  soft.  The  whole  pelt 
on  drying,  therefore  does  not  contract  uniformly  and  the  shrink- 
ing or  curling  up  of  the  pelt  results.  This  leads  to  the  breaking 
of  the  grain  on  bending.  When  much  iron  in  the  tan  liquor  has 
been  reduced  to  the  ferrous  state  and  found  its  way  to  the  pelt, 
it  will  become  oxidized  during  drying.  This  appears  to  be  respon- 
sible for  dark,  wrinkled,  hard  spots  appearing  on  the  grain. 

The  solutions  to  some  of  these  problems  were  found  in  the  use 
of  a  completely  oxidized  iron  tan  liquor;  the  employment  of  a 
small  excess  of  the  oxidizing  agent  in  the  liquor ;  the  maintenance 
of  iron  in  the  ferric  state  by  means  of  an  after  oxidation ;  the  use 
of  optimum  basicity  for  tanning;  and  the  careful  neutralization 
of  the  pelt  after  tanning.  All  these  will  be  dealt  with  at  some 
length  in  the  following  sections. 

The  ferric  sulphate  in  solution  is  unstable  and  liable  to  be  de- 
composed by  hydrolysis  from  a  neutral  or  slightly  alkaline  solu- 
tion, and  is  very  rapidly  precipitated  upon  the  introduction  of  an 
alkali.  Some  attention  has  been  devoted  to  investigating  the  pos- 
sibility of  correcting  this  tendency.  The  use  of  organic  protective 
colloids,  or  of  gums  that  form  mucilages  in  water,  or  of  sub- 
stances that  chemically  combine  with  iron  to  prevent  precipitation 
of  ferric  hydroxide  from  an  alkaline  solution,  entails  many  com- 
plications. The  difficulties  in  such  cases  are  ( I )  that  those  nitro- 
genous protective  colloids  such  as  gelatine,  egg  albumin,  blood 
albumin,  etc.,  that  are  extensively  used  in  connection  with  the 
other  parts  of  leather  manufacture  are  themselves  coagulated  by 
the  highly  concentrated  tan  liquor;  (2)  that  the  poly-hydroxy 
alcohols*  in  the  form  of  syrup  glucose,  gum  dextrin,  starch,  etc., 
exert  a  more  or  less  reducing  action  on  the  ferric  iron  in  the 
liquor,  and  (3)  that  compounds  like  Rochelle  salt  that  hold  up 
the  ferric  iron  in  an  alkaline  solution  yield  no  tannage.f  Other 
gummy  bodies  such  as  Irish  moss,  gum  arabic,  gum  tragacanth, 
etc.,  have  hardly,  any  effect.  The  presence  of  a  chromic  salt  or 
an  aluminum  salt  in  the  iron  liquor  yielding  the  corresponding 
hydroxide  in  an  alkaline  solution  has  some  tendency  to  hold  up 

*  Glycerine  can  be  used  and  seems  to  yield  a  tannage  giving  a  soft, 
red,  transparent  leather. 

fThis  speaks  strongly  of  the  chemical  theory  of  leather  formation  in 
iron  tannage. 


58  LEATHER   CHEMISTS   ASSOCIATION 

the  precipitation  of  iron  as  ferric  hydroxide  and  thus  stabilizes  it, 
especially  when  the  amount  present  is  equal  to  or  greater  than 
that  of  the  iron,  but  this  works  best  in  a  solution  so  alkaline  as 
to  peptize  the  chromic  hydroxide  or  aluminium  hydroxide.30  To 
regulate  the  speed  of  precipitation  and  also  of  the  tanning  action 
of  iron  in  the  pelt  there  is,  at  present,  no  satisfactory  way  except 
by  the  careful  adjustment  of  the  basicity  of  the  iron  liquor  and 
of  the  control  in  subsequent  neutralization. 

To  minimize  the  interference  of  iron  in  the  leather  with  dyeing, 
it  is  found  that  if  the  neutralized  tanned  pelt  is  first  dried  to 
"crust"  before  coloring,  the  iron  appears  better  fixed  in  the  fiber 
and  its  chemical  activity  greatly  lessened.  The  use  of  pyrogallol 
tannins  such  as  sumac,  oak,  etc.,  or  certain  less  astringent  catechol 
tannins  such  as  mimosa,  gambier,  etc.,  then  gives  only  a  light 
grayish  color  so  that  these  tannins  can  be  used  for  mordanting 
as  can  also  other  vegetable  matter  such  as  fustic,  etc.,  that  do  not 
produce  a  decided  black  with  iron  when  used  in  small  quantities. 
To  keep  the  leather  soft  and  flexible  it  is  generally  advisable  to 
apply  a  somewhat  heavy  fat  liquoring,  or  an  oil  re-tan  using 
marine  oils  such  as  cod  liver  oil,  shark  liver  oil,  etc. 

30  Cf.  "Hydrous  Chromic  Oxide"  by  C.  F.  Nagel,  Jr.,  Jour.  Phys. 
Chem.,  Vol.  19,  p.  331  (1915). 

Also  "On  the  Behavior  of  Some  Oxides  with  Caustic  Potash  in  the 
Presence  of  Oxide  of  Chromium"  by  Northcote  and  Church,  Vol.  6,  p.  54 
(1853). 


IRON  TANNAGE  59 

SECTION  IX. .  CHROME-IRON  JOINT  TANNAGE. 

This  form  of  joint  tannage  from  the  use  of  sodium  dichromate 
as  the  oxidizing  agent  for  iron  proved  to  be  very  successful.  It 
did  not  give  a  pure  iron  tannage,  but  a  joint  tannage  of  the  chrome 
as  well  as  iron.  The  relative  amount  of  tannage  due  to  each  in 
the  resulting  leather  is  dependent  upon  the  relative  quantity  of 
each  present  in  the  liquor. 

From  the  invention  of  the  Augustus  Schultz's  two-bath  chrome 
process,  it  has  been  established  that  a  dichromate,  or  chromium  in 
the  hexavalent  state,  has  little  or  no  tanning  property  until  after 
it  is  reduced  to  the  chromic  (trivalent)  state,  Cr+++.  It  was 
seen*  that  ferrous  iron  had  no  tanning  property  until  after  it  was 
oxidized  to  the  ferric  state.  Considering  the  properties  of  the 
two  salts  it  is  evident  that  a  combination  of  the  two  is  a  natural 
outcome,  using  one  as  the  oxidizing  agent  and  the  other  as  the 
reducing  agent,  both  being  benefited  by  the  reaction  mutually  en- 
gaged in  so  that  a  joint  tannage  results.  From  the  chrome  tan- 
nage point  of  view,  the  use  of  the  ferrous  salt  as  a  reducing  agent 
possesses  some  advantages  over  the  other  reducing  agents  such 
as  sodium  bisulphite,  sodium  thiosulphate,  sulphurous  acid,  glu- 
cose, glycerine,  etc.  For,  unlike  these  latter  which  generally  leave 
inert  substances  in  the  bath  after  the  reduction  reaction  and  which 
contribute  nothing  beyond  the  reduction  of  the  chromate,f  the 
ferric  salt  formed  from  the  reduction  reaction  of  the  ferrous  salt 
can  be  utilized  as.  a  tanning  agent  in  the  same  bath.  From  the 
iron  tannage  point  of  view,  the  choice  of  the  dichromate  as  an 
oxidizing  agent  is  prompted  by  many  considerations.  First,  as 
an  oxidizing  agent  its  oxidation  potential  is  high  and  the  oxida- 
tion reaction  rapid,  proceeding  to  completion  in  the  cold.  Second, 
for  its  oxidation  action  it  requires  only  a  very  low  acidityj  in  the 
solution  so  that  the  basicity  of  the  resulting  iron  liquor  is  com- 
pletely under  control.  Third,  the  waste  product  from  the  oxida- 
tion reaction,  namely,  the  chrome  salt,  is  a  valuable  tanning  agent 
which  can  contribute  fully  to  its  share  in  the  resulting  tannage. 
Fourth,  the  green  color  of  the  chrome  tannage  has  the  effect  of 

*  Section  8,  page  54,  footnote. 

t  In  case .  sodium  thiosulphate  is  used  as  the  reducing  agent,  the  col- 
loidal sulphur  may  contribute  some  tannage,  and  it  gives  a  lighter  color 
to  the  leather. 

$  Contrast  the  case  where  a  chlorate  is  used  as  the  oxidizing  agent. 


60  I^ATHDR   CHEMISTS   ASSOCIATION 

neutralizing  the  red-yellow  color  of  the  iron,  yielding  a  product 
of  a  less  pronounced  color.  One  possible  drawback  in  the  use  of 
iron  as  the  reducing  agent  for  the  chromate  might  be  that  the 
quantity  of  the  ferrous  salt  used  is  comparatively  large  (5^2  parts 
of  FeSO47H2O  to  i  part  of  Na2Cr2O7.2H2O  by  weight)  espec- 
ially when  the  commercial  copperas  has  been  partially  air-oxi- 
dized, and  that  the  color  of  the  product  is  somewhat  darker 
(brownish)  than  when  other  reducing  agents  are  used  with  the 
chromate  (light  green).  But  for  a  certain  class  of  goods  this 
is  not  objectionable,  and  advantage  can  well  be  taken  of  the  lower 
cost  of  production.  It  is  important  that  the  basicity  of  the  bath 
be  carefully  adjusted,*  otherwise  the  bath  may  be  either  too  acid 
for  the  chrome  or  too  alkaline  for  the  iron,  so  that  joint  tannage 
can  not  be  brought  about.  In  general,  an  amount  of  66°  Be.  sul- 
phuric acid  equal  to  from  30-35  per  cent,  of  the  copperas  em- 
ployed with  a  sufficient  amount  of  the  sodium  dichromate  for 
complete  oxidation  is  found  to  work  well. 

The  following  procedure  may  illustrate  the  mode  of  tannage 
in  actual  tannery  practice.  The  percentages  given  are  all  calcu- 
lated on  the  basis  of  the  weight  of  the  drained,  pickled  pelt  (sheep- 
skins, calfskins,  etc.).  For  convenience,  the  weight  of  the  skins 
is  taken  as  basis  to  figure  the  quantities  used.  By  "gal.  %"  (gal- 
lons per  cent.)  is  meant  gallons  of  the  liquid  in  question  per  100 
pounds  of  the  skin.  For  goatskins  a  somewhat  larger  quantity 
should  be  taken,  say  10-20  per  cent,  greater.  The  examples  given 
apply  to  drum  tanning. 

Pei  cent. 

(I)   Copperas   (FeSO4.;H2O)    11 

Salt  (NaCl)    5 

Sulphuric  acid   (66°  Be.  H2SO4) i*/2 

Water   for  solution    (total) 12  gal. 

Drum  pelt  in  the  solution  for  ^2  hour,  then  introduce  a  solution  of 

Per  cent. 

Sodium  dichromate   (Na2Cr2O7.2H2O) 2.y$ 

Water  to  dissolve   2  gal. 

*  Historically  Hylten  Cavalin  came  close  to  the  process,  but  because 
of  lack  of  proper  adjustment  for  the  acidity  he  failed  to  obtain  a  success- 
ful tannage  (Section  2,  page  73). 


IRON    TANNAGE  6 1 

Drum  for  about  iJ/£  hours.  (See  if  all  iron  is  oxidized.)  Add 
very  slowly  in  portions,  preferably  through  the  trunnion,  a  solu- 
tion of 

Per  cent. 

Soda  ash   (Na2CO3)    4^ 

Water  to  dissolve 3  gal. 

After  all  the  alkali  is  in,  drum  for  10  minutes  longer.  (See  if  the 
pelt  is  neutralized.)  Rinse.  This  gives  a  tannage  more  of  the 
nature  of  the  iron  than  the  chrome.  The  following  modification 
can  also  be  employed,  if  desired. 

Per  cent. 

(II)   Sodium  dichromate    (Na2Cr2O7.2H2O) 2^ 

Salt  (NaCl) 5 

Water  for  solution   12  gal. 

Drum  the  pelt  in  the  solution  for  y^  hour.  Add  to  the  drum  a 
solution  of 

Per  cent. 

Copperas    (FeSO4.7H2O)    12 

Sulphuric  acid  (66°  Be.  H2SO4) 2^ 

Water  for  solution 4  gal: 

Drum  for  i*/2  hours.  (See  that  all  chrome  is  reduced.)  Run  out 
excess  spent  liquor.  Then  introduce  a  suspension  of 

Per  cent. 

Bleaching  powder    irA 

Water 3  gal. 

Drum  for  15  minutes.  Introduce  very  slowly  as  before  a  solution 
of 

Per  cent. 

Soda  ash 3^2 

Water  to  dissolve 3  gal. 

After  all  alkali  is  in,  drum  for  10  minutes  longer.  (See  if  the 
pelt  is  neutral.)  Rinse.  This  gives  a  tannage  more  of  the  nature 
of  chrome  than  iron. 


62  l^ATHER  CHEMISTS  ASSOCIATION 

(III)  For  one-bath  tannage. 

(a)  When  the  liquor  is  to  be  prepared,  take  for  each  100  pounds 

of  the  pelt 

Per  cent. 

Copperas  (FeSO4.7H2O)   11 

Sulphuric  acid  (66°  Be.  H2SO4) 3 

Salt  (NaCl)    5 

Sodium  dichromate  (Na2Cr2O7.2H3O) 2^4 

Water    (total)    for  solution 15  gal. 

(Add  the  dichromate  very  slowly  when  stirring.  Use  the  liquor 
without  unnecessary  delay.) 

(b)  When  a  concentrated  one-bath  is  already  made  according  to 

the  method  of  preparation  given  in  Section  3,  take 

Per  cent. 

Chrome-iron  liquor   (concentrated) 3  gal. 

Water  to  dilute  .' 12  gal. 

Soda  ash  (Na2CO3)  to  neutralize y* 

In  either  case,  drum  the  pelt  in  the  liquor  for  i  to  ij£  hours,  or 
until  the  pelt  is  struck  through. 

Introduce  very  slowly  as  before 

Per  cent. 

Soda  ash   (Na2CO3)    5 

Water  to  dissolve 4  gal. 

After  all  the  alkali  is  in,  drum  for  10  minutes  longer.  (See  if 
the  pelt  is  neutral.)  Rinse. 

The  stock  tanned  by  any  of  the  above  processes  should  be  soft 
and  full.  It  has  a  color  varying  from  a  dull  yellow,  grayish 
brown,  to  olive  drab,  depending  upon  the  proportion  of  the  chrome 
to  the  iron  present.  To  secure  the  predominating  effect  of  the 
chrome  tannage,  some  chromic  salt  may  be  added  to  the  liquor. 
The  leather  obtained  does  not  stand  boiling  unless  the  chrome 
content  is  increased  by  the  addition  of  a  chromic  salt  to  the  liquor. 

The  leather  can  be  dyed  black  with  logwood  with  or  without  a 
"striker."  It  can  be  dyed  with  coal  tar  dyes,  such  as  the  acid, 
direct,  and  alizarine  dyes.  When  it  is  to  be  dyed  with  a  basic 
dye  a  mordant  is  required,  in  which  case,  the  tanned  stock  is  best 
first  dried  to  "crust"  and  then  wetted  back  for  mordanting  with 
fustic  or  other  vegetable  matter  after  which  the  basic  dye  is 
applied  in  the  usual  manner.  For  fat-liquoring  a  somewhat  larger 


IRON   TANNAGE  63 

quantity  of  the  fat-liquor  (5-8  per  cent,  of  "sulphonated"  cod 
liver  oil,  degras,  " sulphonated"  Neat's  foot  oil,  or  a  commercially 
prepared  mixture)  can  be  used.  The  proper  temperature  for  dye- 
ing is  between  I3O°-I4O°  F.  and  that  for  fat-liquoring  no°- 
120°  F. 

The  leather  can  be  re-tanned  in  oil  to  advantage  when  cod  liver 
oil  or  other  fish  oil  may  be  used.  It  can  be  re- tanned  and  colored 
black  in  ordinary  vegetable  tannins.  Some  basic  black  can  be 
used  for  topping.  When  a  less  astringent  tannin  is  used,  a  light 
gray  color  is  obtained.  In  such  cases,  drying  to  "crust"  prior  to 
the  treatment  is  advisable. 

A  sample  of  sheepskin  leather  tanned  according  to  (I)  above 
gives  the  following  analysis : 

Per  cent. 

Moisture    743 

Ash 15-69 

Fat 21.48 

Fe2O3  10.51 

CraO3 1.84 

*P2O5    2.50 

SO3  (total)    ; 2.12 

Hide  substance  (N  X  5.62) 43.85 

*  From  some  disodium  phosphate  introduced  together  with  the  alkali 
for  neutralization. 


64  LEATHER   CHEMISTS   ASSOCIATION 

SECTION  X.    PURE  IRON  TANNAGE. 

As  it  is  desired  to  determine  the  actual  tanning  value  of  a  ferric 
salt,  a  considerable  portion  of  the  time  has  been  devoted  to  the 
study  of  the  pure  iron  tannage,  that  is  to  say,  to  the  tannage 
where  no  other  metals  except  iron  that  can  yield  a  tannage  are 
present.  It  has  been  often  reported  that  iron  tannage  produces  a 
brittle  leather,  a  leather  that  draws  together  on  drying,  a  leather 
that  deteriorates  on  keeping,  and  so  on.  One  of  the  arguments 
advanced  is  that  iron  in  the  leather  acts  as  an  oxygen  carrier, 
taking  in  oxygen  from  the  air  and  imparting  it  to  the  fiber,  so 
that  the  fiber  is  gradually  oxidized  and  corroded  in  the  course  of 
time.31  It  has  so  far  not  been  found  possible  to  confirm  this  re- 
port, but,  on  the  other  hand,  there  is  sufficient  evidence  to  show 
that  any  defect  of  this  kind  is  due  to  an  improper  tannage  rather 
than  to  the  inherent  nature  of  the  tannage.  For,  when  a  leather 
is  properly  tanned,  it  is  not  at  all  brittle,  does  not  draw  together 
hard  on  drying,  nor  behave  in  any  way  different  from  other  min- 
eral tannage.  Samples  of  sheepskin  leather  tanned  with  iron  salts 
that  have  now  been  kept  for  more  than  ten  months  show  no  sign 
of  deterioration  of  the  sort  reported.  It  is  probable  that  these 
defects  were  brought  about  by  the  use  of  a  too  alkaline  iron  liquor 
in  which  ferric  oxide  had  been  caused  to  deposit  on  the  surface, 
making  the  interior  of  the  pelt  impenetrable  to  the  tanning  agent. 
Too  rapid  a  neutralization  would  also  cause  the  same  defect,  as 
the  ferric  oxide  which  is  caused  to  separate  from  the  solution 
would  coat  the  surface  of  the  pelt.  This  gives  rise  to  a  hard 
outer  layer  (grain)  and  a  wide  soft  zone  underneath  in  the  cross 
section  of  the  pelt. 

As  it  is  the  ferric  iron  that  possesses  the  tanning  property,  it 
follows  that  all  iron  should  be  kept  in  the  ferric  state.  It  is  not 
so  much,  however,  to  avoid  a  small  loss  of  iron  going  to  the  fer- 
rous state  as  to  prevent  the  ferrous  iron  finding  its  way  to  the 
pelt  causing  irregularities  in  appearance  and  texture  in  the  leather. 
Hence  it  is  necessary  to  use  a  sufficient  quantity  of  a  proper  oxi- 
dizing agent  to  bring  about  complete  oxidation,  and  not  only  that, 
to  use  a  small  excess  of  the  oxidizing  agent  (10-15  per  cent.) 
to  take  care  of  any  subsequent  reduction.  To  insure  this,  a  fur- 
ther guaranty  is  found  in  the  introduction  of  a  small  quantity  of 
31  "Die  moderne  Leder-Fabrikation"  by  Hermann  Zeidler,  p.  109  (1914). 


IRON    TANNAGE  65 

a  suitable  oxidizing  agent  (-Na2Cr2O7.2H2O,  CaClO.Cl,  etc.)  to- 
ward the  end  of  the  tanning  process,  prior  to  the  neutralization — 
the  so-called  after  oxidation.  This  is  a  proper  action  at  this  stage, 
inasmuch  as  the  oxidation  reaction  involves  a  decrease  of  hydro- 
gen ion  concentration  in  the  solution,  thus  helping  to  fix  the  iron 
in  the  pelt  . 

The  best  basicity  for  the  tan  liquor  is  found  to  lie  in  a  range 
varying  between  the  ratio  of  5  equivalents  of  the  mineral  acid 
radical  (or  radicals)  present  to  every  6  equivalents  of  the  ferric 
iron,  and  that  of  3  equivalents  of  the  mineral  acid  radical  (or 
radicals)  present  to  every  4  equivalents  of  the  ferric  iron.  When 
much  iron  salt  in  the  liquor  is  in  the  form  of  a  sulphate,  it  is 
safer  not  to  go  too  near  the  higher  limit  of  basicity.  For  if  such 
is  the  case  a  light  yellow  hydrated  ferric  oxide  (not  a  red  gela- 
tinous ferric  hydroxide)  would  then  be  thrown  out  on  short 
standing.  It  is  evident  that  the  same  danger  of  rapid  precipita- 
tion exists  during  neutralization.  Hence  it  is  necessary  to  effect 
the  neutralization  very  gradually.  The  total  amount  of  an  alkali 
needed  for  neutralization  is  only  70-80  per  cent,  of  the  theoretical. 

For  the  oxidation  of  iron  and  the  preparation  of  the  tan  liquor 
from  a  ferrous  salt,  chlorine  is  found  to  work  very  satisfactorily, 
as  it  can  effect  the  oxidation  in  the  cold  in  the  absence  of  any 
acid  and  push  the  reaction  to  completion  under  a  small  pressure.* 
Other  suitable  methods  of  oxidation  are  those  using  nitric  acid 
and  sulphuric  acid,  and  sodium  nitrate  and  sulphuric  acid.  The 
latter  is  more  economical  because  of  the  cheapness  of  the  sodium 
nitrate  (Chile  saltpetre)  employed.  All  methods  involving  the 
use  of  nitric  acid  in  one  form  or  another,  however,  require  a 
boiling  temperature,  and  hence  a  special  container  to  resist  the 
corrosive  action  of  the  hot  nitric  acid. 

For  tanning,  a  quantity  of  ferrous  sulphate  crystals,  FeSO4.- 
7H2O,  between  12  and  15  per  cent,  of  the  weight  of  the  drained 
pelt  is  generally  sufficient,  the  higher  figure  being  for  heavy  hides 
and  for  the  goat  skin.  A  rough  guide  to  secure  the  correct  bas- 
icity for  tanning  is  to  add  10-14  per  cent,  of  soda  ash,  Na2CO3, 
of  the  weight  of  the  ferrous  sulphate  crystals  taken.  This  pre- 
sumes that  the  ferric  liquor  to  start  with  is  neutral  in  composition. 

*  For  detailed  directions  concerning  the  preparation  of  the  tan  liquor, 
see  Section  3. 


66  LEATHER   CHEMISTS   ASSOCIATION 

The  following  method  for  sheepskins  in  drum  tanning  can  be 
used  for  illustration.  Unless  otherwise  stated,  all  percentages  are 
on  the  basis  of  the  weight  of  the  drained  pelt.  When  the  stock 
to  be  tanned  is  much  below  100  pounds  some  judgment  should  be 
exercised  in  regard  to  the  modification  of  these  percentages. 

Per  cent. 

Iron  liquor  containing  an  amount  of  Fe2O3  equal  to 3j4 

(or  as  FeS<X7H2O  12) 

Salt,  NaCl  4 

Soda  ash,  Na2CO3  i# 

Total  volume  for  tanning 25  gal. 

Drum  for  I  to  1^2  hours.    Introduce  into  the  drum  a  suspension 
containing 

Per  cent. 

Bleaching  powder,  CaClO.Cl I  # 

Water    I  gal. 

Drum  for  15  minutes  longer.    Neutralize  the  pelt  very  gradually 

(in  small  portions)  with  a  solution  of 

Per  cent. 

Soda  ash,  Na2CO3 4 

Water  to  dissolve 3  gal. 

After  the  alkali  is  all  fed  in,  drum  for  10  minutes  longer.    Rinse. 
Hang  the  tanned  pelt  to  dry.    Sammy  back  from  "crust"  and  wet 
thoroughly  for  subsequent  operations. 
For  a  coloring  black,  use 

Per  cent. 

Hematine  crystals   il/t 

Water  to  dissolve  equal  to  twice  the  weight  of  the  wet 
stock 

Make  the  solution  alkaline  with  ammonia,  and  heat  to  130°  F. 
Drum  for  30  minutes  and  then  add  to  the  drum  a  solution  warmed 
to  130°  F.  containing 

Per  cent. 

Basic  leather  black i 

Water  to  dissolve   5  gal. 

.  Drum  for  20  minutes,  or  until  the  leather  is  colored  through. 
Run  off  the  spent  dye  liquor.  Fat-liquor  with  an  emulsion  at  130° 
F.  containing 


IRON  TANNAGE;  67 

Per  cent. 

"Sulphonated"  cod  liver  oil 6 

Water   80 

Drum  for  45  minutes,  or  until  all  fat-liquor  is  taken  up.  Hang 
the  fat-liquored  stock  to  dry  without  setting  out.  Any  commer- 
cial fat-liquor  mixture  can  be  used. 

For  such  a  black  leather,  however,  a  re-tan  in  ordinary  vege- 
table tannins  is  more  economical  and  advantageous,  since  the  veg- 
etable tannin  not  only  gives  a  black  color  but  also  a  tannage  to 
the  leather.  Consequently  the  leather  obtained  is  fuller.  For  such 
purpose,  use  a  tannin  liquor,  warmed  to  110°  F.,  containing,  say, 

Per  cent. 

Quebracho  liquid  extract 15 

Water   120 

or  a  tannin  liquor  having  a  barkometer  reading  of  from  15°  to 
20°  Bk.  Drum  for  i*^  to  2  hours.  This  generally  gives  a  gray- 
ish black  color.  To  obtain  a  deeper  black  color,  top  the  leather 
with  i  per  cent,  basic  leather  black  in  the  usual  manner.  This 
method  of  blacking  dispenses  with  the  logwood  color  and  even 
with  the  "iron  striker."  For  a  light  (silver)  gray  color,  pyro- 
gallol  tannins,  such  as  pure  oakwood  tannin,  sumac,  etc.,  or  a  less 
astringent  catechol  tannin,  such  as  mimosa,  gambier,  etc.,  can  be 
used.  This  vegetable  re-tan,  however,  can  best  be  carried  out  in 
a  paddle. 

In  the  case  of  heavy  leather,  an  oil  re-tan  can  be  applied  to  ad- 
vantage, using,  say, 

Per  cent. 

Degras 8 

"Sulphonated"  cod  oil 4 

Water 12  gal. 

Drum  for  i^  hours  with  the  mixture  warmed  to  130°  F.  (At 
present  shark  liver  oil  is  available  and  can  be  utilized  for  this  oil 
re-tan.)  Or,  the  leather  may  be  stuffed  with  a  mixture  of  stearin, 
tallow,  and  "sulphonated"  cod  oil,  using,  for  example, 

Per  cent. 

Stearin 4 

Tallow   10 

"Sulphonated"  cod  oil   8 


68  LEATHER   CHEMISTS   ASSOCIATION 

Heat  the  fat  mixture  to  150°  F.  in  the  drum,  and  drum  for  about 
y*  hour. 

In  general,  an  iron  tanned  leather  is  tough,  heavy,  but  some- 
what harsh.  Hence  it  is  generally  advantageous  to  give  the 
leather  a  good  far-liquoring,  or  oil  stuffing,  or  oil  re-tan.  It  does 
not  resist  a  boiling  temperature  but  begins  to  contract  at  about 
170°  F.  or  lower. 

For  dyeing  the  iron  tanned  leather  with  basic  dyes,  the  remarks 
made  in  Sections  8  and  9  apply  here  with  a  greater  force. 

A  sheepskin  leather  tanned  with  8  per  cent.  Fe2(SO4)3,  4  per 
cent.  NaCl,  and  1.6  per  cent.  Na2CO3,  subsequently  neutralized 
with  4^  per  cent.  CaCO3,  and  finally  lightly  fat-liquored  with  a 
mixture  of  neat's  foot  oil  and  a  mineral  oil,  gives  the  following 
analysis : 

Per  cent. 

Moisture    14.10 

Fat  ! 5.37 

Ash    20.01 

Fe2O3  : 4.08 

SO3  (total) 3.26 

Hide  substance  51.22  (N  X  5-62) 

This  sample  of  sheepskin  leather  is  tough  and  full,  but  feels  some- 
what harsh.  It  has  a  beautiful  yellow-red  color.  From  the  analy- 
sis of  the  iron  content,  it  seems  that  an  amount  of  iron  as  low 
as  4  per  cent.  Fe2O3  of  the  weight  of  the  air-dried  sample  is 
sufficient  to  give  a  satisfactory  tannage. 

Calcium  carbonate  (or  the  "precipitated  lime")  or  magnesium 
carbonate  is  found  to  be  very  suitable  for  neutralization  in  place 
of,  or  together  with,  soda  ash.  It  is  cheap  and  can  be  used  in 
excess  to  prevent  the  presence  of  any  free  mineral  acid  (H2SO4) 
in  the  leather.  The  calcium  or  magnesium  sulphate  formed  in  the 
leather  during  neutralization,  furthermore,  serves  to  give  weight 
to  the  leather. 

It  might  also  be  mentioned  that  in  a  tannery  where  chrome  or 
vegetable  tannage  is  employed,  the  presence  of  an  iron  salt  is  in- 
compatible with  good  appearance  of  leather  and  all  possible  care 
is  to  be  taken  to  keep  away  any  iron  from  all  liquors.  In  iron 
tannage  this  difficulty  disappears. 


IRON    TANNAGE  69 

SECTION  XL — IRON  PHOSPHATE  TANNAGE. 

As  the  iron-tanned  leather  has  a  pronounced  red-yellow  color 
and  as  iron  is  capable  of  forming  many  colored  bodies  both  with 
the  organic  and  inorganic  substances,  attention  is  naturally  drawn 
to  the  titilization  of  this  chemical  activity  of  iron  for  coloring 
the  leather  by  a  treatment  with  a  substance  which  combines  with 
iron  to  give  the  color.  At  the  same  time,  it  should  be  equally 
possible  to  find  a  substance  that  will  give  with  iron  a  color  lighter 
than  that  of  the  iron  leather  itself.  In  general,  these  combina- 
tions are  in  the  form  of  a  precipitate.  Thus,  Knapp  treated  his 
iron  leather  with  a  fatty  acid  forming  a  yellow  precipitate  of 
"iron  soap"  in  the  leather.32  He  was  not,  however,  so  much  con- 
cerned with 'the  yellowish  brown  color  of  the  "iron  soap"  as  with 
the  fixation  of  iron  in  the  leather  so  that  it  could  not  be  washed 
out.  The  use  of  potassium  ferrocyanide  solution  for  producing 
a  blue  color  has  been  mentioned.33  But  it  is  found  that  the  color 
is  not  fast  and  is  gradually  washed  out,  especially  when  the  leather 
after  such  a  treatment  is  not  immediately  fat-liquored;  and,  if 
the  pelt  is  treated  with  the  ferrocyanide  solution  before  the  iron 
has  been  fixed  in  the  leather,  a  very  poor  tannage  is  obtained. 
Logwood  (for  dull  black  color)  and  fustic  (for  green-black 
color)  coloring  matters  have  been  long  known  in  their  use  with 
iron  (as  a  "striker")  ;  but  while  they  give  a  fast  color,  they  con- 
tribute no  material  tannage.  And  it  is  said  that  much  excess  of 
iron  should  be  avoided  as  it  would  render  the  leather  brittle  and 
liable  to  crack.34  A  soluble  sulphide  or  polysulphide  has  been 
advocated  for  making  black  leather  in  connection  with  the  iron 
tannage  by  O.  Rohm,35  but  there  are  grave  doubts  as  to  its  prac- 
ticability, because  these  alkaline  or  alkaline  earth  sulphides  gen- 
erally have  a  strong  reducing  action,  and  the  black  ferrous  sul- 
phide formed  is  merely  a  filler  and  not  a  tanning  agent  for  the 
pelt. 

It  is  found  feasible,  however,  to  make  the  black  or  grayish 
leather  by  re-tanning  the  iron  leather  in  ordinary  vegetable 
tannins.  This  not  merely  gives  the  color,  but  also  involves  a 

32  British  Patent  No.  2,716  (1861). 

33  "A  Text  Book  of  Tanning,"  by  H.  R.  Procter,  p.  222  (1885). 

34  "Leather  Dressing,"  by  M.  C.  Lamb,  p.  149  (1907). 

35  British  Patent  No.  103,827  (1917). 


70  T.EATHER  CHEMISTS  ASSOCIATION 

further  tanning  action.  The  leather  thus  obtained  partakes  of 
the  characteristics  of  both  the  mineral  and  the  vegetable  tannages. 

A  scheme  of  making  a  light  colored,  or  substantially  white 
leather  with  the  use  of  a  phosphate  has  been  satisfactorily  worked 
out.  The  function  of  the  phosphate  seems  to  be  more  than  pro- 
ducing a  yellowish  white  compound  of  ferric  phosphate  in  the 
leather.  Borax  having  a  property  of  forming  a  light  red  com- 
pound, ferric  borate,  could  be  used,  but  the  quantity  needed  is 
usually  large.  It  is,  however,  suitable  for  neutralization  because 
of  its  mild  alkaline  nature.  It  may  be  added  that  other  white  or 
yellowish  white  compounds  of  iron  are  the  ferric  arsenate,  and 
the  iodate.  Certain  features  about  these  substances,  such  as  the 
poisonous  character,  the  cost,  etc.,  make  their  use*  for  leather 
making  clearly  impracticable. 

The  idea  of  using  a  phosphate  in  connection  with  the  iron  tan- 
nage was  suggested  by  the  fact  that  from  the  colloid  chemistry 
point  of  view  the  phosphate  ion,  being  a  trivalent  negative  ion, 
should  act  favorably  towards  the  fixation  of  iron  in  the  pelt. 
The  mode  of  procedure  is  illustrated  by  the  processes  given  below : 

I.  The  pelt  to  be  tanned  is  drummed  in  the  ferric  iron  liquor 
of  the  same  character  and  basicity  as  for  pure  iron  tannage*  and 
treated  in  the  same  manner  up  to  the  neutralization  of  the  pelt. 
For  neutralization,  use 

Per  cent. 

Sodium  pyrophosphate,  N^PaOi.  ioH2O  4 

Soda  ash,  Na2C93 2^ 

Water  for  solution  20 

or 

Trisodium  phosphate,  Na3PO4. 12H2O 6 

Soda  ash i^ 

Water,  same  as  above 

Introduce  the  solution  slowly  in  the  usual  manner.  It  is  found 
better  to  introduce  the  carbonate  together  with  the  phosphate  than 
to  add  the  phosphate  alone  first.  Pyrophosphate  is  preferred 
because  of  its  high  phosphate  content  for  a  given  weight.  Borax 
may  be  used  together  with  the  phosphate  and  the  carbonate;  in 
which  case  the  amount  of  the  latter  used  should  be  correspond- 
ingly decreased.  This  tannage  gives  a  leather  of  a  light  color. 
Subsequent  treatments  given  in  Sections  IX  and  X  can  be  fol- 
lowed. 

*  See  Section  10  on  Pure  Iron  Tannage. 


IRON    TANNAGE)  JI 

II.  The  phosphate  may  be  directly  added  to  the  ferric  tan 
liquor  giving  a  fine  milky  suspension.  In  this  case,  the  addition 
of  the  alkali  to  bring  about  the  proper  basicity  for  tanning  should 
be  omitted.  For  sheepskins  in  drum  tanning,  use 

Per  cent. 

Ferric  salt  (calculated  as  Fe2(SO4)3) 9 

Sodium  pyrophosphate,  NaiPaOi.  ioH2O 4 

Total  volume  15  gal. 

Dissolve  the  pyrophosphate  in  a  small  quantity  of  water  and  add 
it  to  the  ferric  salt  solution  slowly  with  constant  stirring.  Hav- 
ing stirred  thoroughly,  introduce  it  immediately  into  the  drum 
and  tan  the  pelt  for  3  to  5  hours,  or  until  the  pelt  is  thoroughly 
penetrated.  Neutralize  the  pelt  slowly  with 

Per  cent. 

Soda  ash,  Na2CO3  : 3^ 

Water  to  dissolve  3  gal. 

After  all  the  alkali  is  fed  in,  rinse  and  hang  to  dry.  This  gives 
an  especially  white  leather.  The  process  is  suitable  for  light 
stock,  such  as  glove  leather  and  the  like.  The  penetration  is 
somewhat  slow,  hence  a  longer  drumming  is  needed.  When  dried, 
the  leather  is  full  and  soft.  It  becomes  velvety  after  staking  and 
perching. 

The  leather  can  be  finished  by  any  of  the  usual  treatments.  In 
dyeing,  with  basic  coal  tar  dyes,  the  ordinary  precautions  in  con- 
nection with  the  use  of  vegetable  mordants  should  be  observed. 
As  the  leather  does  not  resist  a  high  temperature,  it  is  important 
not  to  use  a  temperature  above  140°  F.  in  dyeing  or  fat-liquoring. 

A  sample  of  sheepskin  leather  treated  in  accordance  with  (II) 
above,  but  without  neutralization  or  fat-liquoring,  gives  a  soft 
and  almost  white  leather.  Its  chemical  analysis  gives 

Per  cent. 

Moisture    1 1.48 

Fat  : 11.75 

Ash   12.23 

Fe2O3  3-97 

P2O5    2.32 

S03  (total)    2.15 

Hide  substance  (N  X  5.62) 54-9O 

Here  again  it  shows  that  about  4  per  cent,  of  iron  as  Fe2O3  on 
the  basis  of  the  air-dried  sample  is  sufficient  to  give  a  tannage  for 
light  skins. 


72  LEATHER  CHEMISTS   ASSOCIATION 

SECTION  XII. — CONCLUSIONS. 

The  character  of  the  iron  tannage  seems  to  lie  between  that 
of  the  alum  and  that  of  the  chrome  tannage.  Iron  seems  to 
yield  a  more  permanent  tannage  (towards  water)  than  alum, 
but  like  the  alum  tannage,  iron  tanned  leather  does  not  resist  the 
boiling  temperature  of  water.  If  we  take  the  critical  temperature 
as  that  at  which  the  sample  under  water  begins  to  shrink  or  to 
draw  together  under  the  influence  of  heat,  that  point  generally 
lies  between  i6o°-i75°  F.  In  the  case  of  a  re-tanned  leather  (in 
fish  oils  or  vegetable  tannins)  a  somewhat  higher  test  may  be 
obtained ;  but  in  no  case  can  an  iron-tanned  leather  stand  boiling, 
unless  considerable  portion  of  the  tannage  is  due  to  chrome  as  in 
the  case  of  the  chrome-iron  joint  tannage. 

It  has  been  often  reported  that  iron-tanned  leather  produces  a 
brittle  grain,  and  rots  on  storing.  To  do  justice  to  the  iron  tan- 
nage it  must  be  declared  that  an  iron-tanned  leather,  properly 
tanned,  is  not  brittle  on  the  grain  and  does  not  deteriorate  on 
storage.  Samples  of  the  leather  that  have  now  been  kept  for 
more  than  ten  months  show  no  sign  of  deterioration.  Sometimes 
the  product  obtained  is  somewhat  stiff  and  "flat,"  but  this  should 
not  be  ascribed  to  the  inherent  properties  of  the  tannage.  The 
strength,  the  fullness,  the  elasticity  are,  in  our  opinion,  a  matter 
of  proper  tannage  and  not  dependent  upon  the  nature  of  the 
tannage. 

As  a  considerable  amount  of  salt  (4-5  per  cent,  of  the  weight 
of  the  pelt)  is  needed  in  the  liquor  and  much  of  it  is  formed  from 
neutralization,  it  is  important  to  rinse  the  tanned  stock  after  neu- 
tralization to  wash  off  most  of  the  neutral  salts  present  (NaCl, 
Na2SO4,  etc.)  ;  otherwise  their  presence  in  the  leather  may  cause 
dampness  or  even  salt  stains  or  spues.  Iron  tannage  is  much 
affected  by  the  presence  of  grease  or  any  imperfections  in  the 
skins,  and  when  such  is  the  case,  unevenness  of  color  and  other 
irregularities  are  liable  to  show  up  on  drying.  Hence  the  neces- 
sity of  uniform  softening  of  the  pelt  and  of  degreasing. 

Iron-tanned  leather  generally  runs  high  in  ash.  The  leather 
has  oftentimes  a  harsh  feel,  due  probably  to  the  presence  of  a 
large  amount  of  iron  oxide  (Fe2O3)  in  the  leather.  Because  of 
the  harsh  feel  it  is  generally  advisable  to  give  the  leather  a  some- 


IRON  TANNAGE;  73 

what  heavy  fat-liquoring  or  an  oil  treatment.  The  use  of  flour, 
egg  yolk,  etc.,  may  be  practiced,  if  desired.  At  the  present  stage 
of  our  knowledge  it  seems  that  to  produce  a  satisfactory  tannage 
at  least  for  a  light  leather  an  amount  of  iron  calculated  as  Fe2O3 
not  less  than  4  per  cent,  of  the  weight  of  the  air-dried  sample 
should  be  present. 

The  iron-tanned  leather  compares  favorably  with  other  mineral 
tanned  leather.  The  red-yellow  or  brown-red  color  of  the  tan- 
nage, however,  is  for  some  purposes  an  undesirable  feature.  The 
chemical  activity  of  iron  in  the  leather  forming  dark  colored  com- 
pounds in  the  leather  is  another  drawback.  But  even  with  all 
these  limitations,  there  is  much  to  be  said  in  its  favor.  There 
are  certain  classes  of  goods  in  which  these  features  are  of  no 
consequence  and  the  saving  in  the  cost  of  production  is  very 
material.  True,  there  are  difficulties  in  connection  with  the  tan- 
ning operation  and  subsequent  treatment  of  the  leather — difficul- 
ties which  in  other  tannages  either  do  not  exist,  or  are  less  serious. 
But  the  process,  like  any  other  new  process,  necessitates  a  new 
set  of  conditions.  To  summarize,  the  following  main  factors  may 
be  mentioned : 

I.  Completeness  in  the  oxidation  of  iron  and  maintenance  in 
its  ferric  state  by  using  an  excess  of  a  proper  oxidizing  agent,  and 
by  means  of  an  after  oxidation. 

II.  Adjustment  of  proper  basicity  by  the  addition  of  a  proper 
amount  of  an  alkali,  a  basicity  between  the  ratio  of  one  OH~ 
equivalent  to  every  5  equivalents  of  the  mineral  acid  radical  pres- 
ent, and  that  of  one  OH~  equivalent  to  every  3  equivalents  of 
the   mineral   acid   radical   present,   being  the  proper   range   for 
tanning. 

III.  Gradual  neutralization  to  be  effected  so  that  iron  may  be 
uniformly  fixed  in  the  pelt  throughout  its  thickness. 

IV.  Drying  to  the  "crust"  state  before  subsequent  treatment  to 
minimize  the  chemical  reactions  between  the  iron  in  the  stock 
and 'the  substances  employed  that  would  react  with  iron  to  give 
an  undesirable  color. 

It  shoul^  not  be  omitted  to  mention  that  the  subject  of  iron 
tannage  presents  a  broad  unexplored  field  and  that  this  study  is 
far  from  being  exhaustive.  Other  phases  could  have  been  taken 
up  and  it  is  hoped  that  this  work  will  serve  as  an  indication  for 
much  that  remains  to  be  done. 


74  LEATHER  CHEMISTS  ASSOCIATION 

BIBLIOGRAPHY. 
THE  MORE  IMPORTANT  WORKS  IN  LEATHER  INDUSTRY. 

I.  Books  on  Leather  Manufacture. 

1.  Leather  Dressing,  by  M.  C.  Lamb. 

G.  Sadler  &  Co.,  London. 

2.  Handbuch  der  Chromgerbung,  by  Josef  Jettmar. 

Schulze  &  Co.,  Leipzig. 

3.  Die  Rotlederfabrikation,  by  Joseph  Borgmann. 

M.  Krayn,  Berlin. 

(  I.  Teil,  Die  Unterlederfabrikation.) 
(II.  Teil,  Die  Oberlederf abdication.) 

4.  The  Principles  of  Leather  Manufacture,  by  H.  R.  Procter. 

E.  &  F.  N.  Spon,  Ltd.,  London. 

5.  Die  Chromgerbung,  by  Joseph  Bergmann. 

M.  Krayn,  Berlin. 

6.  Practical  Tanning,  by  Louis  A.  Flemming. 

Henry  Carey  Baird  &  Son,  Philadelphia. 

7.  Modern  American  Tanning   (in  two  volumes). 

Edited  by  Jacobsen  Publishing  Co.,  Chicago. 

8.  Praxis  und  Theorie  der  Leder-Erzeugung,  by  Joseph  Jettmar. 

Julius  Springer,  Berlin. 

9.  The  Manufacture  of  Leather,  by  H.  G.  Bennett. 

Constable  &  Co.,  London. 
10.  Die  moderne  Lederfabrikation,  by  H.  Zeidler. 

Bernh.  Tr.  Voigt,  Leipzig, 
n.  La  Tannerie,  by  Louis  Meunier  and  Clement  Vaney. 

Gauthier-Villars,  Imprimeur. 

Libraire  de  1'Ecole  Polytechnique,  Paris. 

12.  Die  Feinlederfabrikation,  by  Joseph  Bergmann. 

M.  Krayn,  Berlin.  ' 

13.  A  Text  Book  of  Tanning,  by  H.  R.  Procter. 

E.  &  F.  N.  Spon,  Ltd.,  London.    . 
14  Leather,  by  K.  J.  Adcock. 

Sir  Isaac  Pitman  &  Sons,  Ltd.,  London. 

15.  Leather  Worker's  Manual,  by  H.  C.  Standage. 

Scott,  Greenwood  &  Son,  London. 

16.  The  Making  of  Leather,  by  H.  R.  Procter. 

Cambridge  University  Press,  England. 

17.  Leder-Fabrikation,  by  H.  Kronlin. 

Max.  Janecke,  Hannover. 

18.  Leather  Manufacture,  by  Alex.  Watt. 

Crosby,  Lockwood  &  Son,  London. 

19.  The  Manufacture  of  Leather,  by  Chas.  T.  Davis. 

Henry  Carey  Baird  &  Co.,  Philadelphia. 

20.  Laboratory  Guide  of  Industrial  Chemistry,  by  Allen  Rogers. 

D.  Van  Nostrand  Co.,  New  York. 


IRON    TANNAGE  75 

21.  Practical  Treatise  on  the  Leather  Industry,  by  A.  M.  Villon; 

translated  by  F.  T.  Addyman. 
Scott,  Greenwood  &  Son,  London. 

22.  The  Art  of  Tanning,  by  Campbell  Morfit. 

Henry  Carey  Baird  &  Co.,  Philadelphia. 

23.  La  Chimie  du  Cuir,  by  Lion  Eglene. 

H.  Dunod  et  E.  Pinat,  Paris. 

II.  Books  on  Leather  Chemistry. 

1.  Leather  Industries  Laboratory  Book,  by  H.  R.  Procter. 

E.  &  F.  N.  Spon,  Ltd.,  London. 

2.  Leather  Trades'  Chemistry,  by  S.  R.  Trotman. 

Chas.  Griffin  &  Co.,  Ltd.,  London. 

3.  Handbuch  fiir  Gerberei-Chemische  Laboratorien,  by  G.  Grasser. 

Schulze  &  Co.,  Leipzig. 

4.  Leather  Chemists'  Pocket-Book,  by  H.  R.  Procter. 

E.  &  F.  N.  Spon,  Ltd.,  London. 

5.  Practical  Leather  Chemistry,  by  Arthur  Harvey. 

Crosby,  Lockwood  &  Son,  London. 

6.  Tanners'  and  Chemists'  Hand-Book,  by  L.  E.  Levi  and  E.  V. 

Manuel,  Milwaukee. 

///.  Scientific  and  Commercial  Periodicals  of  Leather  Industry. 

1.  JOURNAL  American  Leather  Chemists'  Association. 

Easton,  Pa.,  U.  S.  A. 

2.  The  Leather  Manufacturer. 

Boston,  Mass.,  U.  S.  A. 

3.  Le  Cuir,  Edition  Technique. 

Paris,  France. 

4.  Journal  of  the  Society  of  Leather  Trades'  Chemists. 

London,  England. 

5.  Collegium, 

Haltigen,  Germany. 

6.  Ledertechnische  Rundschau. 

Berlin,  Germany. 

7.  Der  Gerber. 

Prague,  Czecho-Slovakia. 

8.  Color  Trade  Journal. 

New  York,  N.  Y.,  U.  S.  A. 

9.  Shoe  and  Leather  Reporter. 

Boston,  Mass.,  U.  S.  A. 

10.  Hide  and  Leather. 

Chicago,  111.,  U.  S.  A. 

11.  The  Leather  World. 

London,  England. 

12.  Shoe  and  Leather  Journal. 

Toronto,  Canada., 


=* 


76  LEATHER   CHEMISTS   ASSOCIATION 

IV.  Miscellaneous. 

1.  The  Chemical  Constitution  of  the  Proteins   (in  two  parts),  by 

R.  H.  A.  Plimmer. 
Longmans,  Green  &  Co.,  London. 

2.  The  Physical  Chemistry  of  the  Proteins,  by  T.  B.  Robertson. 

Longmans,  Green  &  Co.,  London. 

3.  Die  Gerbstoffe,  by  J.  Dekker. 

Gebriider  Borntaeger,  Berlin. 

4.  The  Puering,  Bating  and  Drenching,  by  J.  T.  Wood. 

E.  &  F.  N.  Spon,  Ltd.,  London. 

5.  Hides  and  Skins. 

Shoe  and  Leather  Weekly,  Chicago,  111. 


IRON    TANNAGE  77 

APPENDIX. — A  TENTATIVE  PROCEDURE  FOR  THE  ORDINARY 
CHEMICAL  ANALYSIS  OF  IRON-TANNED  LEATHER. 

With  more  new  chemicals  introduced  in  the  manufacture  of 
leather,  the  chemical  analysis  of  the  leather  naturally  becomes 
more  complicated.  The  following  is  a  proposed  system  of  the 
chemical  analysis  for  iron-tanned  leather  ordinarily  sufficient  for 
commercial  work.  With  the  exception  of  the  determination  of 
free  mineral  acid,  all  procedures  here  given  have  been  tested 
and  found  to  give  satisfactory  results.  While  there  are  but  few 
features  in  these  methods,  the  details  of  the  procedure,  and  the 
quantities  of  the  reagents  to  be  taken,  etc.,  are  those  actually 
found  to  work  well.  The  determination  of  the  free  mineral  acid 
is  based  on  the  Procter  and  Searle's  method,  and  that  of  the  hide 
substance  is  adapted  from  the  Dyer's  modification  of  the  Kjeldahl 
method  for  nitrogen.  In  order  to  bring  out  certain  points  in 
the  analysis  more  clearly,  notes  have  been  added  to  each  pro- 
cedure, based  upon  the  results  of  observations  in  the  laboratory. 
The  order  and  the  grouping  of  the  determinations  as  found  to 
be  convenient  are  shown  as  follows : 

(1)  Moisture      >       -n  Qne  sample 

(2)  Fat  ) 

(3)  Ash        1 

(4)  Fe2O3   .  1-       in  one  sample 

(5)  Cr,03      j 

(6)  Free  mineral  acid      j 

(7)  P2O5  }•       in  one  sample 

(8)  SO,  (total)  j 

(9)  Hide  substance 

When  only  isolated  determinations  are  desired,  this  order,  of 
course,  need  not  be  followed. 

Sampling. — Leather  to  be  analyzed  should  be  reduced  .to  small 
pieces  of  approximately  uniform  size.  Heavy  leather  can  be 
shaved  with  a  planer  and  ground  in  a  small  mill.  Light  leather 
should  be  chipped  or  shredded  to  pieces  of  about  %  inch  long  by 
1/16  inch  wide  with  the  natural  thickness  of  the  skin.  A  com- 
posite sample  should  be  made  from  different  parts  of  the  whole 
piece  and  the  sample  intimately  mixed  before  a  portion  is  taken 
for  analysis.  The  prepared  sample  should  be  kept  in  a  tightly 
stoppered  bottle. 


78  LEATHER   CHEMISTS   ASSOCIATION 

1.  Moisture. — Weigh  8  grams  of  the  air-dried  sample  into  a 
tared  glass  dish  and  dry  for  8  hours  in  an  electric  oven  regulated 
at  99°-ioi°  C.    The  loss  in  weight  represents  moisture. 

,   loss  in  wt.  x 

(Per  cent,  moisture  =  100  X  r-.) 

wt.  sample 

NOTE  I. — The  leather  should  not  be  exposed  to  a  higher  tempera- 
ture or  heated  for  an  unnecessary  length  of  time  because 
any  drying  oil  (cod  liver  oil,  shark  liver  oil,  etc.)  used  for 
fat-liquoring,  oiling,  stuffing,  or  re-tanning  would  be  oxi- 
dized to  a  greater  extent.  This  not  only  gives  low  result 
for  moisture,  but  also  for  fat  determination,  as  petroleum 
ether  will  not  dissolve  the  oxidized  fat. 

NOTE  2. — The  dried  sample  should  be  weighed  rapidly  as  it  quickly 
absorbs  moisture  from  the  air. 

2.  Fat. — Transfer  the  sample  from  the  moisture  determination 
to  a  Soxhlet  extractor  using  petroleum  ether  (redistilled  if  neces- 
sary using  distillate  below  60°  C.).     Fill  the  dry,  clean  Soxhlet 
flask  with  160-180  cubic  centimeters  petroleum  ether  (to  about 
three-fourths  full).    Heat  the  flask  in  an  electric  heater  (or  over 
a  water  bath)  for  8  hours  after  which  distill  the  main  portion  of 
the  ether  from  the  flask  into  the  thimble  chamber,  collecting  this 
portion.    Transfer  the  ether  solution  of  the  fat  to  a  tared  evapo- 
rating dish,  evaporate  off  most  of  the  ether  over  a  steam  bath,  and 
dry  the  fat  at  99°-ioi°  C.  in  an  electric  ove'n  for  2  hours.    The 
content  of  the  dish  is  fat. 

(Per  cent,  fat  =. oo  X    wt  of  fat  .) 
wt.  sample  ' 

NOTE  i. — To  prepare  the  thimble  for  extraction,  wash  the  thimble 
(S.  &  S.)  in  a  small  portion  of  the  ether.  Line  the  bottom 
of  this  thimble  with  a  tuft  of  absorbent  cotton  that  has 
also  been  washed  in  the  ether.  Place  the  sample  in  the 
thimble  and  cover  it  with  another  tuft  of  the  washed 
cotton.  This  size  of  the  sample,  together  with  the  cotton 
lining  and  covering  will  be  just  comfortably  contained  in 
the  thimble.  The  thimble  prepared  in  this  way  will  prevent 
any  fine  particles  of  leather  from  being  sucked  out  through 
the  bottom  during  syphoning,  or  from  floating  off  the  top 
when  the  thimble  is  completely  covered  by  the  ether.  A 
piece  of  heavy  glass  tubing  can  be  placed  at  the  bottom 
of  the  chamber  underneath  the  thimble  to  allow  some 
clearance  so  that  the  ether  can  be  completely  drained  from 
the  thimble  during  syphoning.  A  small  tuft  of  cotton  may 


IRON    TANNAGE  79 

be  loosely  placed  at  the  opening  of  the  condenser  above. 

NOTE  2. — The  same  precaution  in  drying  given  in  Note  I  under 
moisture  determination  applies  here.  When  mineral  oil  is 
present  in  the  fat  extraction  it  is  sometimes  difficult  to 
get  a  constant  weight  due  probably  to  the  mineral  oil  being 
constantly  decomposed  and  volatilized  off.  The  dried  fat 
should  also  be  weighed  rapidly. 

NOTE  3. — There  is  only  a  trace  of  iron  salt  that  is  extracted  from 
the  leather  by  the  ether. 

3.  Ash. — Weigh  2  grams  of  the  air-dried  sample  in  a  tared 
platinum  dish,  platinum  crucible,  or  porcelain  crucible  and  heat 
first  very  gently  and  then  to  below  dull-red  heat.     Stir  the  con- 
tents occasionally  with  a  platinum  wire  and  heat  gently  until  it 
is  thoroughly  ashed.    The  residue  is  weighed  as  ash. 

wt.  of  ash    N 

(Percent,  ash  =  100  X  — : p.) 

wt.  sample 

NOTE  I. — Chlorides  of  metals  are  likely  to  be  partially  volatilized 
and  lost  at  a  higher  temperature.  Sulphates  of  heavy 
metals  are  decomposed  with  the  evolution  of  SO8  fumes. 
If  the  sample  is  heated  too  strongly,  especially  at  the 
beginning,  the  leather  cakes  together  so  that  the  inner  part 
is  difficult  to  burn  off.  Sometimes  the  content  fuses  when 
heat  is  applied  too  strongly,  so  that  it  is  hardly  possible 
to  transfer  the  ash  to  a  crucible  for  alkaline  fusion.  In 
this  case  it  is  better  to  use  a  platinum  crucible  for  the  ash 
determination.  But  when  a  phosphate  is  present,  great 
care  should  be  taken  not  to  cause  the  reduction  of  the 
phosphorus  with  the  result  of  ruining  the  platinum  crucible. 

NOTE  2.— For  iron-tanned  leather  a  2-gram  sample  is  sufficient,  as 
the  ash  usually  runs  high. 

NOTE  3.— Owing  to  an  inevitable  loss  of  some  chlorides  and  to  an 
indefinite  amount  of  sulphates  decomposed,  the  significance 
of  the  ash  determination  cannot  be  of  great  value. 
Furthermore,  unless  the  manner  of  heating  and  other  con- 
ditions are  the  same,  good  checks  in  different  hands  are 
difficult. 

4.  Iron.— The  ash  from  the  last  determination  is  fused  in  a 
platinum  crucible  with  a  well  pulverized  and  intimately  stirred 
mixture  containing  i^  grams  anhydrous  pure  K2CO3,  iJ/£  grams 
anhydrous  pure  Na2CO3,  and  i  j£  grams  pure  borax  glass,  until 
the  liquid  in  the  crucible  appears  homogeneous.    Cool,  meanwhile 
heating  to  boiling  150  cubic  centimeters  of  distilled  water  in  a 


8O  LEATHER   CHEMISTS   ASSOCIATION 

350  cubic  centimeter  casserole.  Place  the  crucible  in  the  hot 
water,  cover  the  casserole  with  a  watch  glass,  and  boil  very  care- 
fully. Wash  out  the  contents  of  the  crucible,  break  up  the  mass, 
and  allow  to  settle.  Filter  by  decantation,  and  wash  the  precipi- 
tate with  hot  water,  collecting  the  nitrate  in  a  400  cubic  centi- 
meter beaker.  Ignite  the  precipitate  and  weigh  as  Fe2O3. 

(Per  cent.  Fe2O3  =±£  100  X    wt'  FeA  ^ 

wt.  sample 

NOTE  i. — Only  a  trace  of  iron  is  found  to  pass  into  the  filtrate.  If 
desired,  the  precipitate  on  the  filter  can  be  dissolved  with 
20  cubic  centimeters  hot  dilute  HC1  (i  cone.  HC1  :  2  water 
by  vol.)  and  the  ferric  hydroxide  precipitated  again  with 
NH4OH  with  the  addition  of  2  grams  NH4C1.  Or,  the 
iron  in  the  HC1  solution  can  be  determined  by  the  Zimmer- 
mann-Reinhardt  volumetric  method,  taking  care  to  oxidize 
off  all  organic  matter  with  KMnO4  before  SnCl2  reduction. 

5.  Chromium. — Cool  the  nitrate  from  the  iron  determination 
and  make  it  up  to  250  cubic  centimeters.  Pipette  100  cubic  centi- 
meters of  the  nitrate  into  a  500  cubic  centimeter  beaker.  Dilute 
to  about  200  cubic  centimeters.  Acidify  with  concentrated  HC1 
and  add  5  cubic  centimeters  in  excess.  Add  15  cubic  centimeters 
of  15  per  cent.  KI  solution  and  titrate  with  N/io  sodium  thio- 
sulphate  solution,  adding  i  cubic  centimeter  of  thin,  clear  starch 
solution  after  the  color  of  the  solution  has  changed  from  red  to 
light  yellow.  Titrate  to  the  disappearance  of  the  blue  color. 

(Per  cent.  Cr.O,  =  :oo  X  <*•  N/io  NaAO3  X  °.oo2533  x       }_ 

wt.  sample 

NOTE  i. — When  the  chromium  present  is  small,  the  orange  color  of 
the  dichromate  cannot  be  distinguished.  Hence  the  acidi- 
fication should  be  guided  by  a  litmus  paper. 

NOTE  2. — A  thin,  clear  starch  solution  that  can  keep  for  several 
months  is  prepared  as  follows:  Take  i  gram  ordinary 
starch  powder  and  rub  it  into  a  paste  with  25  cubic  centi- 
meters distilled  water.  Heat  200  cubic  centimeters  dis- 
tilled water  to  boiling  and  stir  the  thin  paste  into  the  hot 
water.  Boil  for  a  few  minutes  when  a  transparent  solu- 
tion will  be  obtained.  Filter  the  solution  through  absorbent 
cotton  into  a  250  cubic  centimeter  glass  stoppered  bottle. 
Add  5  cubic  centimeters  chloroform,  stopper  the  bottle  and 
shake. 

NOTE  3. — When  chromium  in  the  iron  tan   liquor  is  to  be  deter- 


IRON    TANNAGE  8 1 

mined,  pipette  25  cubic  centimeters  of  the  sample  in  a 
250  cubic  centimeter  graduated  flask.  Make  up  to  the 
mark.  Take  25  cubic  centimeters  and  dilute  to  35  cubic 
centimeters  with  distilled  water.  Oxidize  the  chromium 
with  Na2O2  by  adding  small  portions  at  a  time  with  con- 
stant shaking ;  i  ^  to  3  grams  Na2O2  is  sufficient  for  a 
sample  containing  15  to  30  milligrams'  Cr2O3.  After  all 
the  Na2O2  has  been  added,  heat  the  solution  until  the  vol- 
ume remaining  is  about  10  cubic  centimeters.  Add  25  cubic 
centimeters  distilled  water  and  evaporate  down  to  this 
same  volume  again.  Dilute  to  about  150  cubic  centimeters, 
bring  to  a  boil,  allow  to  settle  and  filter  off  the  FeaOt, 
collecting  the  filtrate  in  a  350  cubic  centimeter  beaker. 
Wash  the  precipitate  with  hot  distilled  water,  collecting 
it  with  the  filtrate.  Determine  chromium  in  the  filtrate  as 
before.36  Ignite  the  precipitate  and  weigh  as  Fe2Os. 

(Remark:  Excess  of  Na2Oa  used  must  be  completely 
decomposed  or  a  phenomena  of  the  reappearance  of  the 
starch  blue  color  shortly  after  it  is  discharged  will  occur, 
making  the  determination  worthless.)37 

6.  Free  Mineral  Acid  (Based  on  Procter  and  Searle's  Method). 
— Weigh  2-gram  sample  in  a  platinum  dish.  Cover  the  sample 
with  25  cubic  centimeters  N/io  Na2CO3  (accurately  titrated 
against  the  HC1  used  below).  Allow  the  sample  to  wet  thoroughly 
and  evaporate  to  dryness  on  a  water  bath.  Gently  char  the  or- 
ganic matter,  cover  with  about  50  cubic  centimeters  hot  distilled 
water,  stir,  and  break  up  the  mass.  Filter  into  a  250  cubic  cen- 
timeter beaker.  Return  the  residue  with  the  filter  to  the  dish, 
and  ignite  gently.  Cool  and  take  up  the  ash  with  25  cubic  centi- 
meters N/io  HC1.  Filter  into  the  previous  filtrate  and  wash 
thoroughly.  Add  1-2  drops  methyl  orange,  and  if  red  color  is 
seen,  titrate  with  N/io  alkali. 

(Per  cent,  free  mineral  acid  — 

cc.  N/io  NaOH  X   0.0049  QC  TT  ^n   N 
100  X  — —  — \ —  —  as  H2SO4.) 

wt.  sample 

NOTE  i. — This  method  has  not  been  tested.  A  full  discussion  is 
found  in  "Leather  Industries  Laboratory  Book"  by  H.  R. 
Procter,  pp.  367-73  (1919).  From  the  experience  with 
chrome  leather  analysis,  we  found  that  sometimes  the  color 
of  the  filtrate  was  so  dark  that  it  interfered  with  the 
methyl  orange  color  in  titration. 
w  See  Section  4,  p.  30,  footnote. 

37  See   "On  the  Volumetric   Determination   of   Chromium   in   Chrome 
Leather"  by  Te-Pang  Hou,  JOUR.  Am.  Lea.  Chem.  Assoc.,  p.  367  (1920). 


82  I^ATHSR  CHEMISTS   ASSOCIATION 

7.  Phosphate.  —  Ignite  the  residue  from  the  last  determination 
together  with  the  filter  paper  in  a  platinum  crucible.  Fuse  with 
I  gram  pure  anhydrous  K2CO3,  i  gram  pure  anhydrous  Na2COs, 
and  i  gram  pure  borax  glass  until  the  liquid  appears  homogeneous. 
Dissolve  out  the  content  of  the  crucible  as  described  under  iron 
determination  above,  filter  and  wash,  combining  the  filtrate  with 
the  solution  from  the  alkali  titration  for  Free  Mineral  Acid. 
Acidify  the  solution  slightly  with  HC1.  Add  2  grams  NH4C1 
and  15  cubic  centimeters  magnesia  mixture  and  heat  to  boiling. 
Cool  in  ice  water  and  add  ammonia  very  slowly  at  first  until  the 
solution  smells  of  ammonia  on  stirring.  Add  one-fifth  of  the 
volume  of  the  solution  of  concentrated  ammonia  and  allow  to 
stand  at  room  temperature  for  about  30  minutes.  Filter  by  de- 
cantation  and  wash  with  water  to  which  2-3  per  cent,  ammonia 
has  been  added.  Ignite  the  precipitate  in  a  tared  porcelain 
crucible  first  gently  and  then  strongly  with  a  blast  burner  or  a 
large  burner.  Weigh  as  Mg2P2O7. 


(Per  cent.  P.O.  =  .00  X  wt"  Mg'P'°'  X,  a6376.) 

wt.  sample 

NOTE  i.  —  It  has  been  found  that  precipitating  MgNEkPCX  in  a  hot 
solution  gives  a  purer  precipitate  of  this  composition.  See 
that  a  crystalline  but  not  a  milky  precipitate  results  on  the 
addition  of  ammonia.38 

8.  Total  Sulphate.  —  Acidify  the  filtrate  from  the  phosphate 
determination  with  concentrated  HC1  and  add  5  cubic  centimeters 
in  excess.  Boil  to  expel  CO2.  Add  15  cubic  centimeters  N 
BaCl.,  solution  very  slowly  with  constant  stirring.  Allow  to  stand 
in  a  warm  place  for  from  2  to  4  hours.  Filter  by  decantation  and 
wash  thoroughly  with  hot  water.  Ignite  and  weigh  as  BaSO4. 

(Percent.  SO8  =  100  X    ^t.  BaSO.  X  0.3429 

wt.  sample 

NOTE  i.  —  The  total  sulphate  determination  by  the  alkaline  treatment 
and  fusion  includes  the  free  sulphuric  acid  (if  any),  the 
neutral  sulphates,  and  the  sulphate  from  the  oxidation  of 
sulphur  in  the  protein  substance  (if  no  SO3  is  lost).  With 
sufficient  alkali  present  and  with  slow  heating,  no  material 
amount  of  SO3  should  be  volatilized  during  heating. 

88  Compare  "Analytical  Chemistry,"  Vol.  II,  by  F.  P.  Treadwell,  trans- 
lated by  Hall,  p.  434  (1919). 


IRON    TANNAGE  83 

NOTE  2. — If  the  total  sulphate  alone  is  to  be  determined  the  follow- 
ing method  proves  to  be  convenient  and  satisfactory. 

Weigh  a  2-gram  air-dried  sample  in  a  platinum  crucible, 
cover  it  with  15  cubic  centimeters  of  N/5  Na2CO3  (approx- 
imately) and  allow  the  sample  to  be  thoroughly  soaked  in 
the  alkali.  Evaporate  to  dryness  in  an  air  bath  (made  by 
setting  the  crucible  into  a  hole  cut  in  a  piece  of  asbestos 
board  so  that  about  one-fifth  of  the  height  of  the  crucible 
projects  above  the  board,  and  placing  the  board  with  the 
crucible  on  an  iron  crucible  of  about  50  cubic  centimeters 
capacity).  When  the  sample  is  thoroughly  dried,  gently 
char  it  over  a  very  low  flame.  Fuse  the  charred  sample 
with  2.Y-2.  grams  K2CO3,  2^2  grams  Na2CO3,  and  2^2.  grams 
borax  (all  chemically  pure)  after  mixing  them  thoroughly. 
Dissolve  out  the  mass  as  described  above.  (Fe2Os  can  be 
determined  here  in  the  precipitate.)  Acidify  the  filtrate 
with  concentrated  HC1,  boil  to  expel  CO2  and  determine 
SO*—  by  BaCl2  in  the  usual  manner. 

Remark :  Other  methods  commonly  recommended  for 
the  total  sulphate  determination  are  based  on  the  destruc- 
tion of  the  organic  matter  by  oxidation  with  (i)  fuming 
nitric  acid  (Stiasny's  method),  (2)  chromic  acid  (a  di- 
chromate  and  concentrated  H2SO4)  and  (3)  sodium  per- 
oxide. With  fuming  nitric  acid  and  chromic  it  is  very 
difficult  to  bring  the  sample  into  solution  even  by  a  pro- 
longed digestion.  With  the  Na2Oa  fusion,  the  method  is 
more  rapid,  but  it  is  accompanied  with  certain  disadvan- 
tages. First,  that  an  iron  or  a  nickel  crucible,  in  place 
of  the  platinum  crucible,  must  be  used  and  this  interferes 
with  the  iron  determination  if  it  is  to  be  made  here;  and 
second,  that  the  frothing  and  spattering  of  the  liquid  dur- 
ing fusion  is  inevitable  (unless  the  sample  is  first  charred, 
in  which  case  some  SOs  would  be  lost).  The  above  method, 
as  described,  permits  the  use  of  the  platinum  crucible ;  gives 
a  very  quiet  fusion  yielding  a  low-fusing  and  non-viscous 
melt;  and  loses  very  little,  if  any,  sulphur  through  vola- 
tilization during  charring  if  sufficient  alkali  (Na3COs)  is 
present  in  the  crucible.  On  the  other  hand,  too  much 
alkali  will  cause  disintegration  of  the  sample  yielding  a 
thick,  frothing  liquid  which  takes  a  long  time  to  dry.  The 
method  is  somewhat  longer  than  the  peroxide  fusion. 
9.  Hide  Substance  (Adapted  from  Dyer-Kjeldahl  Method  for 
the  Nitrogen  Determination). — Weigh  a  i-gram  sample,  wrap  it 
in  a  small  quantitative  filter  paper,  and  introduce  it  into  a  dry 
Kjeldahl  flask  (250  cubic  centimeters  capacity).  Cover  the 
sample  with  25  cubic  centimeters  chemically  pure  1.84  sulphuric 


84  lyEATHER   CHEMISTS   ASSOCIATION 

acid  and  place  about  0.7  gram  mercury  or  0.9  gram  solid  HgSO4 
in  the  flask.  Clamp  the  flask  at  an  inclination  of  about  60°  and 
heat  very  gently  until  no  frothing  is  seen  and  the  black  liquid  boils 
quickly.  Cool  and  introduce  15  grams  anhydrous  Na2SO4  and 
three  glass  beads.  Heat  until  the  sample  is  completely  dissolved 
and  the  color  becomes  light  yellow.  Cool  completely  and  add  very 
carefully  about  150  cubic  centimeters  freshly  distilled  water,  and 
shake  until  all  is  dissolved.  Cool  in  running  water.  Transfer 
the  solution  to  a  250  cubic  centimeter  graduated  flask  and  make 
up  to  the  mark.  Pipette  out  100  cubic  centimeters  into  a  750  cubic 
centimeter  R.  B.  flask,  add  J/£  gram  sodium  sulphide  crystals, 
Na2S  .9H2O,  dissolved  in  a  little  water,  and  allow  to  settle.  Place 
in  the  flask  three  glass  beads  and  three  pieces  of  pumice  stone. 
Make  up  the  volume  to  about  300  cubic  centimeters  with  freshly 
distilled  water.  Dissolve  10  grams  NaOH  in  about  35  cubic  cen- 
timeters water  to  which  is  added  a  small  amount  of  rosolic  acid. 
Pour  the  concentrated  NaOH  solution  into  the  flask  quietly  down 
the  side  without  disturbing.  Connect  the  flask  with  a  Hopkins 
distilling  head  to  a  L,iebig  condenser  and  distill  with  the  delivery 
tubing  dipped  into  the  bottom  of  a  500  cubic  centimeter  Erlen- 
meyer  flask  containing  50  cubic  centimeters  N/io  HC1  (accurately 
standardized),  50  cubic  centimeters  water  and  1-2  drops  methyl 
orange.  Distill  for  about  45  minutes,  when  about  150  cubic  centi- 
meters of  the  water  will  have  passed  over.  Titrate  the  excess  of 
HC1  with  N/io  NaOH. 

(Per  cent,  hide  substance  = 

cc.  HC1  used  up  X  0.00786  N 

100  X  — j—    — X  2.5). 

wt.  sample 

NOTE  i. — Introducing  the  sample  into  the  Kjeldahl  flask  by  wrapping 
it  first  with  a  small  quantitative  filter  paper  prevents  any 
fine  particles  of  the  sample  from  sticking  to  the  upper  part 
of  the  neck. 

NOTE  2. — It  is  necessary  not  to  introduce  NazSQ*  into  the  flask  until 
the  frothing  has  completely  subsided,  otherwise,  trouble- 
some foaming  on  subsequent  heating  will  result.  With 
the  above  proportion  of  HzSCX  and  Na2SO4  and  with  the 
catalytic  effect  of  mercury,  the  sample  will  be  brought  into 
a  complete  solution  in  20-30  minutes. 


IRON    TANNAGE  85 

NOTE  3. — Mercury  is  first  precipitated  as  grayish  HgS  in  an  acid 
solution,  because  it  will  react  with  NH3  yielding  the  mer- 
curic ammonia  chloride  precipitate  Hg.NH2Cl  from  which 
NH3  cannot  be  readily  liberated.  The  precipitate  of  HgS 
does  not  interfere  with  the  distillation,  and  quiet  boil- 
ing prevails  throughout.  With  NaOH  introduced  in  the 
manner  described,  the  heavier  NaOH  solution  will  remain 
at  the  bottom  layer  and  there  is  no  danger  of  loss  of  any 
ammonia  before  distillation.  The  presence  of  an  excess 
of  NaOH  is  indicated  by  the  purple  color  of  the  rosolic 
acid. 

NOTE  4. — The  presence  of  much  chloride  in  the  sample  will  cause 
some  loss  of  NH3  during  HuSO* — 'Na2SO4  digestion,  as 
NH4C1  is  liable  to  volatilize  off  with  strong  heating. 


VITA 

Te-Pang  Hou  was  born  in  August,  1890,  in  Foochow, 
China.  He  obtained  his  primary  education  in  that  locality  and 
later  in  a  Middle  School  in  Foochow  City. 

In  1908  he  entered  the  Railway  Technical  College,  Shang- 
hai, China,  finishing  his  work  in  1910.  He  was  in  engineering 
practice  for  some  time  with  the  Tientsin-Pukow  Railway. 

He  came  to  the  United  States  in  the  fall  of  1913  and 
joined  the  Massachusetts  Institute  of  Technology,  Boston, 
Mass.  There  he  took  up  Chemical  Engineering,  (Course  X) 
and  was  later  admitted  to  the  new  travelling  course  (Course 
X-A).  He  finished  his  work  there  in  1917,  receiving  the 
degree  of  Bachelor  of  Science. 

From  1917-1918  he  was  on  postgraduate  work  in  applied 
leather  chemistry  at  Pratt  Institute,  Brooklyn,  N.  Y.  and  re- 
ceived a  certificate  for  the  work  in  June,  1918.  From  1918- 
1920  he  was  in  the  graduate  school  of  Columbia  University, 
New  York,  N.  Y.,  where  he  received  the  degree  of  Master  of 
Arts  in  June,  1919. 


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